Predefined or built in functions are those functions which comes with PHP by default. But to make some of these functions to work, one need to have some PHP extensions compiled into PHP. Otherwise it will give some fatal errors. For example to use MySql functions to work properly, PHP should be compiled with Mysql support.

It's important to know what a function returns, or if a function works directly on a passed in value etc., to improve the quality of the PHP code.