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.
Function names follow the same rules as other labels in PHP. A valid function name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*.
Tip: You may also want to take a look at the Appendix S.
In PHP 3, functions must be defined before they are referenced. No such requirement exists since 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.
Example 17-2. Conditional functions
All functions and classes in PHP have the global scope - they can be called outside a function even if they were defined inside and vice versa.
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). Both are supported, as of PHP 4: see Variable-length argument lists and the function references for func_num_args(), func_get_arg(), and func_get_args() for more information.
It is possible to call recursive functions in PHP. However avoid recursive function/method calls with over 100-200 recursion levels as it can smash the stack and cause a termination of the current script.