When programming in php and writing to a file, how do you add a newline to the file?

[lisa b]

This is my code to write to a file. I'm trying to add a newline in between each line on the file..
$file = fopen("order.txt","w");
echo fwrite($file,$_POST["tireqty"]);
echo fwrite($file,$_POST["plugqty"]);
echo fwrite($file,$_POST["oilqty"]);
echo fwrite($file,$_POST["address"]);
fclose($file);