$rewards = array('500'=> 80, '1000' => 13, '1500' => 5, '2000' => 1.5, '2500' => 0.4, '20000' => 0.099, '50000' => 0.001);
Do not use float numbers for odds!
You sould multiply the odds:
$rewards = array('500'=> 80000, '1000' => 13000, '1500' => 5000, '2000' => 1500, '2500' => 400, '20000' => 99, '50000' => 1);
Elbandi