In PHP, how do I check if a function exists?

Using function_exists:

if(function_exists('my_function')){
    // my_function is defined
}

http://php.net/manual/en/function.function-exists.php

<?php
  if (!function_exists('myfunction')) {

     function myfunction()
     {
       //write function statements
     }

}
 ?>

var_dump( get_defined_functions() );

DISPLAYS all existing functions

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