Generally, the URL means Uniform Resource Locater. Likewise, URL in PHP Programming Language is also the same. URL is nothing but a website address. It helps in connecting the client and the server when browsed using a specific link.

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.

User-defined functions ¶ 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.

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.