Writing functions

One core element of an extension are functions which are exported to the PHP userland. Even when you're planning to write object-oriented extensions you are advised to read this chapter as most of the information is valid for methods, too.

When adding a function to an extension, for instance after using the ext_skel script to create the raw infrastructure, you can create a function by implementing it as a C function and then providing an entry for the extension's function table. The function entry can contain a pointer to an argument information structure. Providing this information ins not strictly necessary, unless you plan to accept parameters by reference or want to return a reference, but provides information that can be accessed via PHP's Reflection API. As you will see below the parameters aren't passed as direct function parameters to the implementation but passed on a stack, which is checked by the function's implementation which can't directly serve as source for this information.

/* {{{ proto void hello_world()
       Do nothing */
PHP_FUNCTION(hello_world)
{
}
/* }}} */

/* {{{ arginfo_hello_world */
ZEND_BEGIN_ARG_INFO(arginfo_hello_world, 0)
ZEND_END_ARG_INFO()
/* }}} */

/* {{{ demo_functions */
function_entry demo_functions[] = {
    PHP_FE(hello_world, arginfo_hello_world)
    {NULL, NULL, NULL}
}
/* }}} */

/* {{{ demo_module_enry */
zend_module_entry demo_module_entry = {
#if ZEND_MODULE_API_NO >= 20010901
    STANDARD_MODULE_HEADER,
#endif
    "demo",
    demo_functions,
    NULL,
    NULL,
    NULL,
    NULL,
    NULL,
#if ZEND_MODULE_API_NO >= 20010901
    "1.0.0",
#en
    STANDARD_MODULE_PROPERTIES
}
/* }}} */

In this example you can see the above discussed elements and the module structure; if you don't remember this structure go back to The zend_module structure.

The first part of this extension is the actual implementation. As a convention each exported function is preceded by a two line comment describing the function's userland prototype and a short one line comment.

For providing source code compatibility between different versions of PHP the functions declaration is wrapped into the PHP_FUNCTION macro. The compiler's preprocessor will, when using PHP 5.3 transform this into a function like this:

void zif_hello_world(int ht, zval *return_value, zval **return_value_ptr,
                     zval *this_ptr, int return_value_used TSRMLS_DC)
{
}

For preventing a naming conflict between a function exported to userland and another C function with the same name the C symbol of the exported function is prefixed with the prefix zif_. You can also see that this prototype doesn't reference the argument stack. Accessing parameters passed from PHP is described below. The following table lists the C level parameters which are also defined in the INTERNAL_FUNCTION_PARAMETERS macro. Mind that these parameters might change between PHP versions and you should use the provide macros.

As said the function shown above will simply return NULL to the user and do nothing else. The function can be called, from the PHP userland, with an arbitrary number of parameters. A more useful function consists of four parts in general:

1. Declaration of local variables. In C we have to declare locale vars, so we do this at the functions top.

2. Parameter parsing. PHP passes parameters on a special stack: we have to read them from there, verify the types, cast them if needed and bail out if something goes wrong.

3. Actual logic. Do whatever is needed.

4. Set the return value, cleanup, return.

In some cases the exact order of these steps may change. Especially the last two steps are often mixed up, but it's a good idea to stay in that order.

/* {{{ proto void hello_world(string name)
   Greets a user */
PHP_FUNCTION(hello_world)
{
    char *name;
    int name_len;

    if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &name, &name_len) == FAILURE) {
        return;
    }

    php_printf("Hello %s!", name);

    RETURN_TRUE;
}
/* }}} */

The function above shows these parts. Let's start the analysis with the last lines: php_printf is - as one can guess easily - PHP's version of standard C's printf. Unlike printf it doesn't print to the process's STDOUT channel but to the current output stream, which might be buffered by the user. It should be noted that php_printf, unlike most parts of PHP's API is not binary safe, so if name contains a NULL-byte the rest would be ignored. For a binary safe output PHPWRITE should be used.

Nota: In general it is considered to directly print data to the output stream, it is suggested to return data as string to the user instead so the user can decide what to do. Exceptions to this rule might be functions dealing with binary data like images while your API should provide a way to access the data even there.

The RETURN_TRUE macro in the last line does three things: It sets the type of the variable we got as return_value pointer to IS_BOOLEAN and the value to a true value. Then it returns from the C function. So when using this macro you're housekeeping for memory and other resources has to be completed as further code of this function won't be executed.

The zend_parse_parameters() function is responsible for reading the parameters passed by the user from the argument stack and putting it with proper casting in local C variables. If the user doesn't pass the wrong number of parameters or types that can't be casted the function will emit a verbose error and return FAILURE. In that case we simply return from the function - by not changing the return_value the default return value of NULL will be returned to the user.

Nota: Please mind that FAILURE is represented by a -1 and SUCCESS by 0. To make the code clear you should always use the named constants for comparing the value.

The first parameter to zend_parse_parameters() is the number of parameters that have actually been passed to the function by the user. The number is passed as ht parameter to our function but, as discussed above, this should be considered as an implementation detail and ZEND_NUM_ARGS() should be used.

For compatibility with PHP's thread-isolation, the thread-safe resource manager, we also have to pass the thread context using TSRMLS_CC. Unlike in other functions this can't be the last parameter as zend_parse_parameters expects a variable number of parameters - depending on the number of user-parameters to read.

Following the thread context the expected parameters are declared. Each parameter is represented by a character in the string describing the type. In our case we expect one parameter as a string so our type specification is simply "s".

Lastly one has to pass one or more pointers to C variables which should be filled with the variable's value or provide more details. In the case of a string we get the actual string, which will always be NULL-terminated, as a char* and its length, excluding the NULL-byte, as int.

A documentation of all type specifiers and the corresponding additional C types can be found in the file » README.PARAMETER_PARSING_API which is part of the source distribution. The most important types can be found in the table below.