PHP Handbuch
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# Kapitel 32. Accepting Arguments

One of the most important issues for language extensions is accepting and dealing with data passed via arguments. Most extensions are built to deal with specific input data (or require parameters to perform their specific actions), and function arguments are the only real way to exchange data between the PHP level and the C level. Of course, there's also the possibility of exchanging data using predefined global values (which is also discussed later), but this should be avoided by all means, as it's extremely bad practice.

PHP doesn't make use of any formal function declarations; this is why call syntax is always completely dynamic and never checked for errors. Checking for correct call syntax is left to the user code. For example, it's possible to call a function using only one argument at one time and four arguments the next time - both invocations are syntactically absolutely correct.

# Determining the Number of Arguments

Since PHP doesn't have formal function definitions with support for call syntax checking, and since PHP features variable arguments, sometimes you need to find out with how many arguments your function has been called. You can use the `ZEND_NUM_ARGS` macro in this case. In previous versions of PHP, this macro retrieved the number of arguments with which the function has been called based on the function's hash table entry, `ht`, which is passed in the `INTERNAL_FUNCTION_PARAMETERS` list. As `ht` itself now contains the number of arguments that have been passed to the function, `ZEND_NUM_ARGS` has been stripped down to a dummy macro (see its definition in `zend_API.h`). But it's still good practice to use it, to remain compatible with future changes in the call interface. Note: The old PHP equivalent of this macro is `ARG_COUNT`.

The following code checks for the correct number of arguments:

    if(ZEND_NUM_ARGS() != 2) WRONG_PARAM_COUNT;

If the function is not called with two arguments, it exits with an error message. The code snippet above makes use of the tool macro `WRONG_PARAM_COUNT`, which can be used to generate a standard error message (see Abbildung 32-1).

Abbildung 32-1. `WRONG_PARAM_COUNT` in action.

This macro prints a default error message and then returns to the caller. Its definition can also be found in `zend_API.h` and looks like this:

    ZEND_API void wrong_param_count(void);

    #define WRONG_PARAM_COUNT { wrong_param_count(); return; }

As you can see, it calls an internal function named wrong_param_count() that's responsible for printing the warning. For details on generating customized error messages, see the later section "Printing Information."

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