How does php access a variable passed by JQuery?

I have the following code:

$.post("getLatLong.php", { latitude: 500000},
   function(data){
     alert("Data Loaded: " + data);
   });

but I don't understand how to access this value using php.

I've tried

$userLat=$_GET["latitude"]; 

and

$userLat=$_GET[latitude];

and then I try to print out the value (to check that it worked) using:

echo $userLat; but it does not return anything. I can't figure out how to access what is being passed to it.