I would usually format it to seperate the condition from the instructions. In other words:
if ($t=="a1") {
$destination="http://site1.com?sub=";
}else if ($t=="a2") {
$destination="http://site2.com?sub=";
}else {
$destination="http://site3.com=";
}
But it's really just a matter of what is easiest to read. I find my version easiest to debug.