In PHP 3, functions must be defined before they are referenced.
Many people ask how to call functions that are in other files.
The scoping rules should also not be buried in subsection 4 of the variables section. When using a function within a function, using global in the inner function will not make variables available that have been first initialized within the outer function. That error is there to stop a recursive call from completely taking up the entire system memory. I thought it worth to mention. Simple Quick Sort using recursion works perfectly.