PHP Credit Card Number Validation

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;  
}
?>