It is one page with different actions, or get requests.
Basically anything after ? is the variable name, anything after the = is what thait variable contains if you want two variables use &.
and to call those
This should output:
$a = $_GET['a'];
$b = $_GET['b'];
echo "a= $a <br /> b = $b";
unless I typed something wrong you also may want to use isset (http://php.net/manual/en/function.isset.php) so you aren't calling variables that have been set/used.
Corrections to my coding/thoughts welcome.