php: get suffix from a given file name

functions.php

/**
 * get suffix from a given file_name
 *
 * @since  [1.0]
 * @see    [filetype()]
 * @author Richard
 * @date   2015-05-05
 *
 * @param  [string]                      $file_name [a.txt]
 * @return [sring]                                 [txt]
 */
function file_suffix($file_name)
{
    return substr($file_name, strrpos($file_name, '.') - strlen($file_name) + 1);
}