Queries can be sent as either POST or GET. I'll explain how to retrieve both methods.
$POST[] - This is an array containing all the post variables. The array uses names of the variable to retrieve the data they represent. Therefore, when someone queries ?a=b, $POST["a"] = b.
$_GET[] - This is the exact same as above, but only for GET.
Using them by name - Using the above example, $a = b.