Using require_once in a Plugin?

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What’s the difference between these two code snippets, and which one should I use for a WordPress plugin? 

require_once(dirname(__FILE__).'/inc/canagan_admin_functions.php');
require_once( '/inc/canagan_admin_functions.php' );
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  1. The first one is like saying…

    Include the file found in the inc directory, above the directory
    where this file is located.

    The second statement is saying…

    in the server root (/) look in the inc folder for
    canagan_admin_functions.php and include it.

    The first one will work, the second won’t. In the second you’re looking in the root of the entire file system, where you file certainly won’t be.

    As a side note, whenever I start a plugin that’s going to span multiple files, and/or enqueues CSS or JS files, I usually set up a few constant to help me along the way. Here’s an example.

    <?php
define( 'CD_PLUGIN_PATH', plugin_dir_path( __FILE__ ) );
define( 'CD_PLUGIN_URL', plugin_dir_url( __FILE__ ) );

    plugin_dir_path and plugin_dir_url are nice because they always return paths/urls with trailing slashes.

    Then to include/require files, I use the constants. Keeps from having to call dirname(__FILE__) on every include.

    You can then use the constants to include files…

    require_once( CD_PLUGIN_PATH . 'path/to/file.php' );

    or to enqueue scripts/styles…

    <?php
add_action( 'wp_enqueue_scripts', 'wpse35088_enqueue' );
function wpse35088_enqueue()
{
    wp_enqueue_script( 'wpse35088-script', CD_PLUGIN_URL . 'path/to/script.js' );
}