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.
PHP does not support function overloading, nor is it possible to undefine or redefine previously-declared functions.
Note: 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.