Why use if function_exists?

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I noticed a lot of developers of WordPress themes would use this in functions.php

if (function_exists('register_nav_menus')) {
    register_nav_menus (array('primary' => 'Header Navigation'));
}

For me, I alwayse use add_action for every function I am using in functions.php

So, the above would be written as:

add_action('init', 'my_register_nav_menus');

function my_register_nav_menus() {
    register_nav_menus (array('primary' => 'Header Navigation'));
}

I have two questions:

  1. Why use the if statement in the first method?

  2. Which one is the correct way?

5

Checking to see if built in WordPress functions exist before calling them is for backward compatibility which IMHO is not needed.

So if you see if ( function_exists( 'register_nav_menus' ) ) the theme author is supporting versions earlier than 3.0.

You still sometimes see if ( function_exists( 'dynamic_sidebar' ) ) Why? I couldn’t tell you because dynamic_sidebar was introduced in 2.2.

Another reason to use it is to make your theme or plugin pluggable. A pluggable function is one that can be overridden in a child theme or another plugin.

This is done on the definition not the call and you use the ! operator to make sure it doesn’t already exist before you define it.

if ( ! function_exists( 'my_awesome_function' ) ) {
/**
 * My Awesome function is awesome
 *
 * @param array $args
 * @return array
 */
function my_awesome_function( $args ) {
  //function stuff
  return array();
  }
}

When this is done a child theme or other plugin can override that function with there own.

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