PHP Credit Card Number Validation

<?
function validate_cardnum($cardnum)   {  
        $checkdigit=substr($cardnum,-1);  
        $remainingcardnum=substr($cardnum,0,strlen($cardnum)-1);  
        $i=0;  
      
        while($i < strlen($remainingcardnum))  {  
                if($i%2==0)  
                        $remaing_array[$i]=substr($remainingcardnum,($i+1)*-1,1) * 2;  
                else  
                        $remaing_array[$i]=substr($remainingcardnum,($i+1)*-1,1);  
          
                if($remaing_array[$i]>=10)  
                        $checksum=$checksum+1;  
 
                $checksum=$checksum+($remaing_array[$i]%10);  
                $i++;  
        }  
 
        $calculatedcheckdigit=(10-($checksum%10))%10;  
        if($calculatedcheckdigit==$checkdigit)  
                return true;  
        else  
                return false;  
}
?>