Get directory and filename of a plugin, by passing its filename only

function get_dir_and_filename_of_active_plugin( $plugin_filename = 'akismet.php' ) {
	$plugin_dir_and_filename = false;
	$active_plugins          = get_option( 'active_plugins' );
	foreach ( $active_plugins as $active_plugin ) {
		if ( false !== strpos( $active_plugin, $plugin_filename ) ) {
			$plugin_dir_and_filename = $active_plugin;
			break;
		}
	}
	return $plugin_dir_and_filename;
}