把全角数字转为半角数字

//把全角数字转为半角数字
<?
function GetAlabNum($fnum){
$nums = array("０","１","２","３","４","５","６","７","８","９");
//OSPHP.com.CN
$fnums = "0123456789";
        for($i=0;$i<=9;$i++) $fnum = str_replace($nums[$i],$fnums[$i],$fnum); //OsPHP.COM.CN
$fnum = ereg_replace("[^0-9.]|^0{1,}","",$fnum);
        if($fnum=="") $fnum=0;
        return $fnum;
}
?>

这个有什么作用