wp_plugin_directory_constants()

function wp_plugin_directory_constants() {
	if ( !defined('WP_CONTENT_URL') ) {
		define( 'WP_CONTENT_URL', get_option('siteurl') . '/wp-content'); // full url - WP_CONTENT_DIR is defined further up
	}

	/**
	 * Allows for the plugins directory to be moved from the default location.
	 *
	 * @since WP-2.6.0
	 */
	if ( ! defined( 'WP_PLUGIN_DIR') ) {
		define( 'WP_PLUGIN_DIR', WP_CONTENT_DIR . '/plugins' ); // full path, no trailing slash
	}

	/**
	 * Allows for the plugins directory to be moved from the default location.
	 *
	 * @since WP-2.6.0
	 */
	if ( ! defined( 'WP_PLUGIN_URL') ) {
		define( 'WP_PLUGIN_URL', WP_CONTENT_URL . '/plugins' ); // full url, no trailing slash
	}

	/**
	 * Allows for the plugins directory to be moved from the default location.
	 *
	 * @since WP-2.1.0
	 * @deprecated
	 */
	if ( ! defined( 'PLUGINDIR') ) {
		define( 'PLUGINDIR', 'wp-content/plugins' ); // Relative to ABSPATH. For back compat.
	}

	/**
	 * Allows for the mu-plugins directory to be moved from the default location.
	 *
	 * @since WP-2.8.0
	 */
	if ( ! defined( 'WPMU_PLUGIN_DIR') ) {
		define( 'WPMU_PLUGIN_DIR', WP_CONTENT_DIR . '/mu-plugins' ); // full path, no trailing slash
	}

	/**
	 * Allows for the mu-plugins directory to be moved from the default location.
	 *
	 * @since WP-2.8.0
	 */
	if ( ! defined( 'WPMU_PLUGIN_URL') ) {
		define( 'WPMU_PLUGIN_URL', WP_CONTENT_URL . '/mu-plugins' ); // full url, no trailing slash
	}

	/**
	 * Allows for the mu-plugins directory to be moved from the default location.
	 *
	 * @since WP-2.8.0
	 * @deprecated
	 */
	if ( ! defined( 'MUPLUGINDIR' ) ) {
		define( 'MUPLUGINDIR', 'wp-content/mu-plugins' ); // Relative to ABSPATH. For back compat.
	}
}