Source

File: wp-admin/includes/plugin.php

function get_mu_plugins() {
	$wp_plugins   = array();
	$plugin_files = array();

	if ( ! is_dir( WPMU_PLUGIN_DIR ) ) {
		return $wp_plugins;
	}

	// Files in wp-content/mu-plugins directory.
	$plugins_dir = @opendir( WPMU_PLUGIN_DIR );
	if ( $plugins_dir ) {
		while ( ( $file = readdir( $plugins_dir ) ) !== false ) {
			if ( '.php' === substr( $file, -4 ) ) {
				$plugin_files[] = $file;
			}
		}
	} else {
		return $wp_plugins;
	}

	closedir( $plugins_dir );

	if ( empty( $plugin_files ) ) {
		return $wp_plugins;
	}

	foreach ( $plugin_files as $plugin_file ) {
		if ( ! is_readable( WPMU_PLUGIN_DIR . "/$plugin_file" ) ) {
			continue;
		}

		// Do not apply markup/translate as it will be cached.
		$plugin_data = get_plugin_data( WPMU_PLUGIN_DIR . "/$plugin_file", false, false );

		if ( empty( $plugin_data['Name'] ) ) {
			$plugin_data['Name'] = $plugin_file;
		}

		$wp_plugins[ $plugin_file ] = $plugin_data;
	}

	if ( isset( $wp_plugins['index.php'] ) && filesize( WPMU_PLUGIN_DIR . '/index.php' ) <= 30 ) {
		// Silence is golden.
		unset( $wp_plugins['index.php'] );
	}

	uasort( $wp_plugins, '_sort_uname_callback' );

	return $wp_plugins;
}