Explanation

Hi. Im looking over someone else's code and I come across something I dont understand and dont know where in the manual to look for it. Here is an example ==> $string1 = (isset($string2)) ? $HTTP_GET_VARS['pid'] : ""; I have no idea what the '?' in the line and the ':' at the end, so I have no way of interpreting what $string1 could be. If you could point me to the page in the manual that would be great. Thanks...