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.
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.