function foo ($arg_1, $arg_2, ..., $arg_n)
{
    echo "Example function.\n";
    return $retval;
}

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.

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.