50/50 php traffic filter

[Roni20]

Hi

I'm looking for script which will send first clickt to address1 second click to address2 third click to address 1 etc...

I have something like this

PHP Code:

<?
$a=rand(0,1);
if($a==0){
      Header("Location: http://www.google.com");
}
   else{
     Header("Location: http://www.yahoo.com");
}
?>

it's almost ok because here is 50% chance that will be 0 or 1, but here can be 3 times in turn 1 next can be 5 time in turn o etc. I need exactly 50/50.

Can you help ?