php查询文件扩展名

 //查询文件扩展名
 function extension($str){
     $str = implode("",explode("\\",$str));
     $str = explode(".",$str);
     $str = strtolower(end($str));
     return $str;
 }

 $file = 'D://a\b\name-Of_somefile.File.txt';
 echo extension($file);