Functions that are written by the user are User defined functions.

    function function name [(argument1, argument 2, ...argument n)]
	{ 
	  any PHP code
	}

The syntax of a user defined function has a unique function name,the user can pass any number of arguments seperated by comma, the user can include any PHP code and even another function.

    <?php
    function hello() 
     {   
      print("Hello!");  
     }
    ?>

Using the function hello() anywhere in the PHP code will display the word "Hello".Usually functions and classes have global scope,and functions cannot be un-defined or re-defined.