3.1415926535898
The above example is the result of using the PHP built-in function of pi() which returns pi.

Alexis
The above example is the result of creating my own function writename() which returns my name.

Predefined functions in PHP are "built-in" functions that are already reserved in PHP and allow users to quickly utilize them without having write out the function code themselves. User defined functions are created by the user and can be customized to the user's needs.