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 = &#39;&#39;;

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

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

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

    }

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

    return $oknum;

}