In this chapter, we discuss SWIG's support of PHP. Currently any PHP7 or PHP8 release should work.
Support for PHP7 was added in SWIG 3.0.11 and for PHP8 in 4.1.0. Support for PHP5 was removed in SWIG 4.0.0 and support for PHP4 was removed in SWIG 1.3.37. There never was a PHP6 release.
In order to use this module, you will need to have a copy of the PHP include files to compile the SWIG generated C/C++ sources. If you installed PHP from a binary package, you may need to install a "php-dev" or "php-devel" package for these to be installed. You can find out where these files are by running php-config --includes. To use the built PHP module you will need either the php binary or the Apache php module. If you want to build your extension into php directly, you will need the complete PHP source tree available.
To build a PHP extension, run swig using the -php7 option as follows (-php is also supported and currently is an alias for -php7 but prior to SWIG 4.0.0 it was an alias for -php5):
swig -php7 example.i
This will produce 2 files: example_wrap.c and php_example.h. The first file, example_wrap.c contains all of the C code needed to build a PHP extension. The second file, php_example.h contains the header information needed if you wish to statically link the extension into the php interpreter.
If the interface file uses %pragma(php) include=... or %pragma(php) code=... then SWIG will also generate a third file, example.php to contain what these specify. In SWIG < 4.1.0, this third file was always generated as it defined the PHP classes, etc (but this is now done via C code in example_wrap.c) and also contained code to dynamically load the extension (but this used the PHP dl() function, which isn't recommended nowadays).
SWIG can generate PHP extensions from C++ libraries as well when given the -c++ option. The support for C++ is discussed in more detail in section 27.2.6. The generated C++ wrapper will be called example_wrap.cxx. You can specify a different extension for the C++ wrapper using -cppext - e.g. if you want example_wrap.cc use -cppext cc.
The usual (and recommended) way is to build the extension as a separate dynamically loaded module (which is supported by all modern operating systems).
It is also possible to rebuild PHP from source so that your module is statically linked into the php executable/library. This is a lot more work, and also requires a full rebuild of PHP to update your module, and it doesn't play nicely with package system. We don't recommend this approach, or provide explicit support for it.
To build your module as a dynamically loadable extension, use compilation commands like these (if you aren't using GCC, the commands will be different, and there may be some variation between platforms - these commands should at least work for Linux though):
gcc `php-config --includes` -fpic -c example_wrap.c example.c
gcc -shared example_wrap.o example.o -o example.so
To test the extension from a PHP script, you first need to tell PHP to load it. Assuming you're using PHP 7.2 or higher, the recommended (and simplest!) way to do this is to copy it to PHP's default extension directory and add a line like this to the [PHP] section of php.ini:
extension=modulename
PHP < 7.2 doesn't support loading by just the module name, so you need to specify the filename of the module to be specified, which varies between platforms. And for any PHP version, if the module is not in PHP's default extension directory, you also need to specify the path, for example:
extension=/path/to/modulename.so
If you're using the PHP CLI SAPI it's possible (but not recommended) to use the dl() function to load an extension at run time, by adding a line like this to the start of each PHP script which uses your extension:
dl("/path/to/modulename.so"); // Load the module
But to do this portably you need to take into account that pathnames and the filename extension vary by platform, and for security reasons PHP no longer supports dl() when running PHP through a webserver. Overall it's better to instead use extension in php.ini as described above.
It is important to understand that PHP uses a single global namespace into which all symbols from extension modules are loaded. It is quite possible for names of symbols in one extension module to clash with other symbols unless care is taken to %rename them. At present SWIG doesn't have support for generating wrappers which make use of PHP's namespace feature.
These work in much the same way as in C/C++. Constants can be defined by using either the normal C pre-processor declarations, or the %constant SWIG directive. These will then be available from your PHP script as a PHP constant, (i.e. no dollar sign is needed to access them.) For example, with a swig interface file like this,
%module example #define PI 3.14159 %constant int E = 2.71828
you can access the constants in your PHP script like this,
echo "PI = " . PI . "\n"; echo "E = " . E . "\n";
There's one peculiarity of how constants work in PHP prior to PHP 8 which it is useful to note (this is not specific to SWIG though) - if you try to use an undeclared constant, PHP will emit a warning (or a notice in PHP 7.1 and earlier) and then expand the constant to a string version of the constant's name. Unfortunately it is easy to miss the warning message if you're using PHP in a webserver as it will probably end up in error.log or similar. PHP 8.0 made this an error.
For example,
%module example #define EASY_TO_MISPELL 0
accessed incorrectly in PHP,
if(EASY_TO_MISPEL) {
...
} else {
...
}
The mis-spelled constant will become the string 'EASY_TO_MISPEL', which is treated as true by the if test, when the value of the intended constant would be treated as false!
Because PHP does not provide a mechanism to intercept access and assignment of global variables, global variables are supported through the use of automatically generated accessor functions.
%module example;
%inline %{
double seki = 2;
void print_seki() {
zend_printf("seki is now %f\n", seki);
}
%}
is accessed as follows:
print seki_get(); seki_set( seki_get() * 2); # The C variable is now 4. print seki_get();
SWIG supports global variables of all C datatypes including pointers and complex objects. To support additional types, you just need to supply the standard in and out typemaps, which get used because of the wrapping as _get() and _set() functions.
SWIG honors the %immutable modifier by not generating a _set method (so attempting to call it will give a PHP fatal error). A _get method is still generated so this provides read-only access to the variable from the PHP script.
At this time SWIG does not support custom accessor methods.
C functions are converted into PHP functions. Default/optional arguments are also allowed. An interface file like this :
%module example int foo(int a); double bar(double, double b = 3.0); ...
Will be accessed in PHP like this :
$a = foo(2); $b = bar(3.5, -1.5); $c = bar(3.5); # Use default argument for 2nd parameter
SWIG generates PHP type declarations for function parameters and return types for PHP 8 and later (we don't try to support PHP 7's more limited type declarations and the generated wrappers compiled for PHP 7 will not have any type declarations).
You can control the generation of PHP type declarations using the "php:type" %feature. This has three settings:
If unset or set to "0" then no type declarations are generated, e.g.: %feature("php:type", "0");
If set to "1" then type declarations are generated for both parameters and return types, e.g.: %feature("php:type", "1");
The default setting is "compat", which is the same as "1" except no return type declarations are generated for virtual methods for which directors are enabled. This provides better compatibility for PHP subclasses of wrapped virtual methods in existing SWIG-generated bindings, e.g.: %feature("php:type", "compat");
If you have an existing PHP interface and are upgrading to SWIG >= 4.1.0 then the default "compat" setting should work well.
If you're writing a new set of bindings and only targeting PHP8 or newer then enabling type declarations everywhere probably makes sense. It will only actually make a difference if you enable directors and are wrapping C++ classes with virtual methods, but doing it anyway means you won't forget to if the code you are wrapping later evolves to have such classes and methods.
The type declaration information will make the generated source code and compiler extension module larger, so you might want to turn off type declarations if keeping these small is important to you. If you find you need to turn off type declarations to fix a problem, please let us know via our github issue tracker.
Note that being a SWIG feature this can be specified globally (like above) or per class, per method, etc. See the %feature directives section for full details of how to control at a fine-grained level.
The PHP type information is specified via a "phptype" attribute on "in" and "out" typemaps, and these have been added for all the typemaps we supply for PHP. We don't currently support this for "argout" templates, but probably will in a future version.
If you have written custom SWIG typemaps for PHP and want to add PHP type declarations, then the syntax is very like how you'd specify the type in PHP code, e.g. %typemap(in, phptype="int|string|Foo") means the typemap accepts a PHP int or string or an object of class Foo, %typemap(in, phptype="?int") means a PHP int or NULL, etc. As well as the standard PHP type declaration types, SWIG also understands the special type "SWIGTYPE" as an entry in phptype, which means the PHP type corresponding to the type that this typemap matched on - for a object this will give you the PHP class for the object, and for a pointer to a non-class type it will give you the name of the PHP class SWIG created for that pointer type.
Although PHP does not support overloading functions natively, swig will generate dispatch functions which will use %typecheck typemaps to allow overloading. This dispatch function's operation and precedence is described in Overloaded functions and methods.
Since SWIG 4.1.0, SWIG wraps C/C++ classes directly with PHP objects. Pointers to other types are also wrapped as PHP objects - mostly this is an implementation detail, but it's visible from PHP via is_object() and similar. In earlier SWIG versions, PHP resources were used to wrap both classes and pointers to other types.
There are multiple ways to wrap pointers to simple types. Given the following C method:
void add( int *in1, int *in2, int *result);
One can include cpointer.i to generate PHP wrappers to int *.
%module example %include "cpointer.i" %pointer_functions(int, intp) void add( int *in1, int *in2, int *result);
This will result in the following usage in PHP:
<?php $in1=copy_intp(3); $in2=copy_intp(5); $result=new_intp(); add( $in1, $in2, $result ); echo "The sum " . intp_value($in1) . " + " . intp_value($in2) . " = " . intp_value( $result) . "\n";
An alternative would be to use the include typemaps.i which defines named typemaps for INPUT, OUTPUT and INOUT variables. One needs to either %apply the appropriate typemap or adjust the parameter names as appropriate.
%module example %include "typemaps.i" void add( int *INPUT, int *INPUT, int *OUTPUT);
This will result in the following usage in PHP:
<?php $in1 = 3; $in2 = 5; $result= add($in1, $in2); # Note using variables for the input is unnecessary. echo "The sum $in1 + $in2 = $result\n";
Because PHP has a native concept of reference, it may seem more natural to the PHP developer to use references to pass pointers. To enable this, one needs to include phppointers.i which defines the named typemap REF.
In case you write your own typemaps, SWIG supports an attribute called byref: if you set that, then SWIG will make sure that the generated wrapper function will want the input parameter as a reference.
%module example %include "phppointers.i" void add( int *REF, int *REF, int *REF);
This will result in the following usage in PHP:
<?php $in1 = 3; $in2 = 5; $result = 0; add($in1, $in2, $result); echo "The sum $in1 + $in2 = $result\n";
It is important to note that a php variable which is NULL when passed by reference would end up passing a NULL pointer into the function. In PHP, an unassigned variable (i.e. where the first reference to the variable is not an assignment) is NULL. In the above example, if any of the three variables had not been assigned, a NULL pointer would have been passed into add. Depending on the implementation of the function, this may or may not be a good thing.
We chose to allow passing NULL pointers into functions because that is sometimes required in C libraries. A NULL pointer can be created in PHP in a number of ways: by using unset on an existing variable, or assigning NULL to a variable.
SWIG wraps C++ structs and classes with PHP classes. Since SWIG 4.1.0, this is done entirely via PHP's C API - earlier SWIG versions generated a PHP wrapper script which defined proxy classes which called a set of flat functions which actually wrapped the C++ class.
This interface file
%module vector
class Vector {
public:
double x, y, z;
Vector();
~Vector();
double magnitude();
};
struct Complex {
double re, im;
};
Would be used in the following way from PHP:
<?php $v = new Vector(); $v->x = 3; $v->y = 4; $v->z = 5; echo "Magnitude of ($v->x, $v->y, $v->z) = " . $v->magnitude() . "\n"; $v = NULL; # destructor called. $c = new Complex(); $c->re = 0; $c->im = 0; # $c destructor called when $c goes out of scope.
Member variables and methods are accessed using the -> operator.
SWIG/PHP used to support a -noproxy option to flatten the class structure and generate collections of named flat functions. This is no longer supported as of SWIG 4.1.0.
The constructor is called when new Object() is used to create an instance of the object. If multiple constructors are defined for an object, function overloading will be used to determine which constructor to execute.
Because PHP uses reference counting, simple assignment of one variable to another such as:
$ref = $v;
causes the symbol $ref to refer to the same underlying object as $v. This does not result in a call to the C++ copy constructor or copy assignment operator.
One can force execution of the copy constructor by using:
$o_copy = new Object($o);
Destructors are automatically called when all variables referencing the instance are reassigned or go out of scope. The destructor is not available to be called manually. To force a destructor to be called the programmer can either reassign the variable or call unset($v)
Static member variables in C++ are not wrapped as such in PHP as it does not appear to be possible to intercept accesses to such variables. Therefore, static member variables are wrapped using a class function with the same name, which returns the current value of the class variable. For example
%module example
class Ko {
static int threats;
};
would be accessed in PHP as,
echo "There have now been " . Ko::threats() . " threats\n";
To set the static member variable, pass the value as the argument to the class function, e.g.
Ko::threats(10); echo "There have now been " . Ko::threats() . " threats\n";
Static member functions are supported in PHP using the class::function() syntax. For example
%module example
class Ko {
static void threats();
};
would be executed in PHP as
Ko::threats();
PHP supports the concept of abstract interfaces which a class can implement.
Since SWIG 3.0.3, you can tell SWIG that a wrapped class (for example
MyIterator) implements the Iterator interface like
so:
%typemap("phpinterfaces") MyIterator "Iterator"
If there are multiple interfaces, just list them separated by commas.
Historically PHP has supported dynamic class properties and SWIG has implemented them too (because we implement the magic __get(), __set() and __isset() methods we need to include explicit handling).
PHP 8.2 deprecates dynamic class properties - initially they'll warn, and apparently they'll not work by default in PHP 9.0.
In PHP code dynamic properties can be enabled for a class by marking that class with the attribute #[AllowDynamicProperties].
To follow this PHP change, as of SWIG 4.1.0 you now need enable dynamic properties for any classes you want to support them. To enable for class Foo:
%feature("php:allowdynamicproperties", 1) Foo;
or to enable them for all wrapped classes:
%feature("php:allowdynamicproperties", 1);
Note that unknown features are ignored, so you can add use these unconditionally in your interface file and it'll work with older SWIG too.
You can get SWIG to generate an "example.php" file by specifying the code to put in it using the code pragma.
%module example %pragma(php) code=" # This code is inserted into example.php echo \"example.php execution\\n\"; "
Results in the following in "example.php"
# This code is inserted into example.php echo "example.php execution\n";
The version pragma can be used to add version to generated PHP extension module. The version is inserted in the zend_module_entry block.
%module example %pragma(php) version="1.5"
The include pragma is a short cut to add include statements to the example.php file.
%module example %pragma(php) code=" include \"include.php\"; " %pragma(php) include="include.php" // equivalent.
The phpinfo pragma inserts code in the PHP_MINFO_FUNCTION which is called from PHP's phpinfo() function.
%module example;
%pragma(php) phpinfo="
zend_printf("An example of PHP support through SWIG\n");
php_info_print_table_start();
php_info_print_table_header(2, \"Directive\", \"Value\");
php_info_print_table_row(2, \"Example support\", \"enabled\");
php_info_print_table_end();
"
To insert code into the PHP_MINIT_FUNCTION, one can use either %init or %minit.
%module example;
%init {
zend_printf("Inserted into PHP_MINIT_FUNCTION\n");
}
%minit {
zend_printf("Inserted into PHP_MINIT_FUNCTION\n");
}
To insert code into the PHP_MSHUTDOWN_FUNCTION, one can use either %shutdown or %mshutdown.
%module example;
%mshutdown {
zend_printf("Inserted into PHP_MSHUTDOWN_FUNCTION\n");
}
The %rinit and %rshutdown statements are very similar but insert code into the request init (PHP_RINIT_FUNCTION) and request shutdown (PHP_RSHUTDOWN_FUNCTION) code respectively.
Proxy classes provide a more natural, object-oriented way to access extension classes. As described above, each proxy instance has an associated C++ instance, and method calls to the proxy are passed to the C++ instance transparently.
This arrangement is asymmetric in the sense that no corresponding mechanism exists to pass method calls down the inheritance chain from C++ to PHP. In particular, if a C++ class has been extended in PHP (by extending the proxy class), these extensions will not be visible from C++ code. Virtual method calls from C++ are thus not able access the lowest implementation in the inheritance chain.
Changes have been made to SWIG 1.3.18 to address this problem and make the relationship between C++ classes and proxy classes more symmetric. To achieve this goal, new classes called directors are introduced at the bottom of the C++ inheritance chain. Support for generating PHP classes has been added in SWIG 1.3.40. The job of the directors is to route method calls correctly, either to C++ implementations higher in the inheritance chain or to PHP implementations lower in the inheritance chain. The upshot is that C++ classes can be extended in PHP and from C++ these extensions look exactly like native C++ classes. Neither C++ code nor PHP code needs to know where a particular method is implemented: the combination of proxy classes, director classes, and C wrapper functions takes care of all the cross-language method routing transparently.
The director feature is disabled by default. To use directors you must make two changes to the interface file. First, add the "directors" option to the %module directive, like this:
%module(directors="1") modulename
Without this option no director code will be generated. Second, you must use the %feature("director") directive to tell SWIG which classes and methods should get directors. The %feature directive can be applied globally, to specific classes, and to specific methods, like this:
// generate directors for all classes that have virtual methods
%feature("director");
// generate directors for the virtual methods in class Foo
%feature("director") Foo;
You can use the %feature("nodirector") directive to turn off directors for specific classes or methods. So for example,
%feature("director") Foo;
%feature("nodirector") Foo::bar;
will generate directors for the virtual methods of class Foo except bar().
Directors can also be generated implicitly through inheritance. In the following, class Bar will get a director class that handles the methods one() and two() (but not three()):
%feature("director") Foo;
class Foo {
public:
Foo(int foo);
virtual void one();
virtual void two();
};
class Bar: public Foo {
public:
virtual void three();
};
then at the PHP side you can define
class MyFoo extends Foo {
function one() {
print "one from php\n";
}
}
For each class that has directors enabled, SWIG generates a new class that derives from both the class in question and a special Swig::Director class. These new classes, referred to as director classes, can be loosely thought of as the C++ equivalent of the PHP proxy classes. The director classes store a pointer to their underlying PHP object. Indeed, this is quite similar to struct swig_object_wrapper which is used to implement the PHP proxy classes.
For simplicity let's ignore the Swig::Director class and refer to the original C++ class as the director's base class. By default, a director class extends all virtual methods in the inheritance chain of its base class (see the preceding section for how to modify this behavior). Virtual methods that have a final specifier are unsurprisingly excluded. Thus the virtual method calls, whether they originate in C++ or in PHP via proxy classes, eventually end up in at the implementation in the director class. The job of the director methods is to route these method calls to the appropriate place in the inheritance chain. By "appropriate place" we mean the method that would have been called if the C++ base class and its extensions in PHP were seamlessly integrated. That seamless integration is exactly what the director classes provide, transparently skipping over all the messy extension API glue that binds the two languages together.
In reality, the "appropriate place" is one of only two possibilities: C++ or PHP. Once this decision is made, the rest is fairly easy. If the correct implementation is in C++, then the lowest implementation of the method in the C++ inheritance chain is called explicitly. If the correct implementation is in PHP, the Zend API is used to call the method of the underlying PHP object (after which the usual virtual method resolution in PHP automatically finds the right implementation).
Now how does the director decide which language should handle the method call? The basic rule is to handle the method in PHP, unless there's a good reason not to. The reason for this is simple: PHP has the most "extended" implementation of the method. This assertion is guaranteed, since at a minimum the PHP proxy class implements the method. If the method in question has been extended by a class derived from the proxy class, that extended implementation will execute exactly as it should. If not, the proxy class will route the method call into a C wrapper function, expecting that the method will be resolved in C++. The wrapper will call the virtual method of the C++ instance, and since the director extends this the call will end up right back in the director method. Now comes the "good reason not to" part. If the director method were to blindly call the PHP method again, it would get stuck in an infinite loop. We avoid this situation by adding special code to the C wrapper function that tells the director method to not do this. The C wrapper function compares the called and the declaring class name of the given method. If these are not the same, then the C wrapper function tells the director to resolve the method by calling up the C++ inheritance chain, preventing an infinite loop.
One more point needs to be made about the relationship between director classes and proxy classes. When a proxy class instance is created in PHP, SWIG creates an instance of the original C++ class and stores it in the struct swig_object_wrapper. This is true whether or not directors are enabled for the particular class in question. However when a class derived from a proxy class is created, SWIG instead creates an instance of the corresponding C++ director class. The reason for this difference is that user-defined subclasses may override or extend methods of the original class, so the director class is needed to route calls to these methods correctly. For unmodified proxy classes, all methods are ultimately implemented in C++ so there is no need for the extra overhead involved with routing the calls through PHP.
Memory management issues are slightly more complicated with directors than for proxy classes alone. PHP instances hold a pointer to the associated C++ director object, and the director in turn holds a pointer back to the PHP object. By default, proxy classes own their C++ director object and take care of deleting it when they are garbage collected.
This relationship can be reversed by calling the special ->thisown property of the proxy class. After setting this property to 0, the director class no longer destroys the PHP object. Assuming no outstanding references to the PHP object remain, the PHP object will be destroyed at the same time. This is a good thing, since directors and proxies refer to each other and so must be created and destroyed together. Destroying one without destroying the other will likely cause your program to segfault.
Here is an example:
class Foo {
public:
...
};
class FooContainer {
public:
void addFoo(Foo *);
...
};
$c = new FooContainer(); $a = new Foo(); $a->thisown = 0; $c->addFoo($a);
In this example, we are assuming that FooContainer will take care of deleting all the Foo pointers it contains at some point.
With directors routing method calls to PHP, and proxies routing them
to C++, the handling of exceptions is an important concern. By default, an
exception thrown in PHP code called from C++ causes the PHP interpreter
to flag that an exception is thrown, then return passes to C++ as if
the PHP function had returned Null. Assuming the directorout
typemaps handle this (those SWIG defines by default should) then once
control returns to PHP code again, the PHP exception will actually propagate.
Sometimes this control flow is problematic, and you want to skip any handling in the C++ code. To achieve this, it is necessary to temporarily translate the PHP exception into a C++ exception. This can be achieved using the %feature("director:except") directive. The following code should suffice in most cases:
%feature("director:except") {
#if SWIG_VERSION >= 0x040100
if ($error != NULL)
#else
if ($error == FAILURE)
#endif
{
throw Swig::DirectorMethodException();
}
}
If you only need to support SWIG >= 4.1.0, you can just use the ($error != NULL) condition.
In SWIG 4.1.0, $error was changed in the SWIG/PHP director implementation to make it work more like how it does for other languages. Previously, $error didn't actually indicate an exception, but instead was only set to FAILURE if there was a problem calling the PHP method. Now $error indicates if the PHP method threw a PHP exception, and directorout typemaps for PHP no longer need to be gated by if (EG(exception)).
This code will check the PHP error state after each method call from a director into PHP, and throw a C++ exception if an error occurred. This exception can be caught in C++ to implement an error handler. Currently no information about the PHP error is stored in the Swig::DirectorMethodException object, but this will likely change in the future.
It may be the case that a method call originates in PHP, travels up to C++ through a proxy class, and then back into PHP via a director method. If an exception occurs in PHP at this point, it would be nice for that exception to find its way back to the original caller. This can be done by combining a normal %exception directive with the director:except handler shown above. Here is an example of a suitable exception handler:
%exception {
try { $action }
catch (Swig::DirectorException &e) { SWIG_fail; }
}
The class Swig::DirectorException used in this example is actually a base class of Swig::DirectorMethodException, so it will trap this exception. Because the PHP error state is still set when Swig::DirectorMethodException is thrown, PHP will register the exception as soon as the C wrapper function returns.
Enabling directors for a class will generate a new director method for every virtual method in the class' inheritance chain. This alone can generate a lot of code bloat for large hierarchies. Method arguments that require complex conversions to and from target language types can result in large director methods. For this reason it is recommended that you selectively enable directors only for specific classes that are likely to be extended in PHP and used in C++.
Compared to classes that do not use directors, the call routing in the director methods does add some overhead. In particular, at least one dynamic cast and one extra function call occurs per method call from PHP. Relative to the speed of PHP execution this is probably completely negligible. For worst case routing, a method call that ultimately resolves in C++ may take one extra detour through PHP in order to ensure that the method does not have an extended PHP implementation. This could result in a noticeable overhead in some cases.
Although directors make it natural to mix native C++ objects with PHP objects (as director objects) via a common base class pointer, one should be aware of the obvious fact that method calls to PHP objects will be much slower than calls to C++ objects. This situation can be optimized by selectively enabling director methods (using the %feature directive) for only those methods that are likely to be extended in PHP.
Typemaps for input and output of most of the basic types from director classes have been written. These are roughly the reverse of the usual input and output typemaps used by the wrapper code. The typemap operation names are 'directorin', 'directorout', and 'directorargout'. The director code does not currently use any of the other kinds of typemaps. It is not clear at this point which kinds are appropriate and need to be supported.
Director typemaps for STL classes are mostly in place, and hence you should be able to use std::string, etc., as you would any other type.