php代码中全角数字如何转半角

function GetAlabNum($fnum){

 $nums = array('０','１','２','３','４','５','６','７','８','９','．','－','＋','：');

 $fnums = array('0','1', '2','3', '4','5', '6', '7','8', '9','.', '-', '+',':');

 $fnlen = count($fnums);

 for($i=0;$i<$fnlen;$i++) $fnum = str_replace($nums[$i],$fnums[$i],$fnum);

 $slen = strlen($fnum);

 $oknum = '';

 for($i=0;$i<$slen;$i++){

 if(ord($fnum[$i]) > 0x80) $i++;

 else $oknum .= $fnum[$i];

 }

 if($oknum=="") $oknum=0;

 return $oknum;

}