检索插件或mu-plugins目录下的一个URL。

如果没有提供参数，则默认为插件目录中的URL。

function plugins_url( $path = '', $plugin = '' ) {

	$path          = wp_normalize_path( $path );
	$plugin        = wp_normalize_path( $plugin );
	$mu_plugin_dir = wp_normalize_path( WPMU_PLUGIN_DIR );

	if ( ! empty( $plugin ) && 0 === strpos( $plugin, $mu_plugin_dir ) ) {
		$url = WPMU_PLUGIN_URL;
	} else {
		$url = WP_PLUGIN_URL;
	}

	$url = set_url_scheme( $url );

	if ( ! empty( $plugin ) && is_string( $plugin ) ) {
		$folder = dirname( plugin_basename( $plugin ) );
		if ( '.' !== $folder ) {
			$url .= '/' . ltrim( $folder, '/' );
		}
	}

	if ( $path && is_string( $path ) ) {
		$url .= '/' . ltrim( $path, '/' );
	}

	/**
	 * Filters the URL to the plugins directory.
	 *
	 * @since 2.8.0
	 *
	 * @param string $url    The complete URL to the plugins directory including scheme and path.
	 * @param string $path   Path relative to the URL to the plugins directory. Blank string
	 *                       if no path is specified.
	 * @param string $plugin The plugin file path to be relative to. Blank string if no plugin
	 *                       is specified.
	 */
	return apply_filters( 'plugins_url', $url, $path, $plugin );
}