A function is a piece of code that takes another input in the form of a parameter, processes it, and then returns a value. A PHP Function feature is a piece of code that can be used over and over again and accepts argument lists as input, and returns a value. PHP comes with thousands of built-in features.

PHP User Defined Functions A function is a block of statements that can be used repeatedly in a program. A function will not execute automatically when a page loads.

This is done using an ampersand operator [ & ] before the parameter name. Passing by reference also means that you have to provide a variable as an argument when invoking the function or PHP will throw an error.

By default, function arguments are passed by value (so that if the value of the argument within the function is changed, it does not get changed outside of the function). To allow a function to modify its arguments, they must be passed by reference. $str = 'This is a string, '; add_some_extra($str);