A function may be defined using syntax such as the following:
Any valid PHP code may appear inside a function, even other functions and class definitions.
In PHP 3, functions must be defined before they are referenced. No such requirement exists in PHP 4. Except when a function is conditionally defined such as shown in the two examples below.
When a function is defined in a conditional manner such as the two examples shown. Its definition must be processed prior to being called.
Пример 13-1. Conditional functions
PHP does not support function overloading, nor is it possible to undefine or redefine previously-declared functions.
Замечание: Function names are case-insensitive, though it is usually good form to call functions as they appear in their declaration.
PHP 3 does not support variable numbers of arguments to functions, although default arguments are supported (see Default argument values for more information). PHP 4 supports both: see Variable-length argument lists and the function references for func_num_args(), func_get_arg(), and func_get_args() for more information.