Adding a random parameter to your request will not sabotage your php in any way:
loader = new URLLoader();
var request:URLRequest =
new URLRequest(
"http://www.mySite.com/getTime.php?action=getTime&bogus="+(Math.random() * 10000));
loader.load(request);
Your php code will GET the action
parameter, and will ignore the bogus
parameter without any effect on your code.
You can also use a time based random number to avoid being too unlucky and get the same value twice.