I think you should re-read what he is asking for. The code you provided:
$random = $possible[array_rand($possible)];
That is a 33.33% chance for any of the amounts.
Your method is 33.33% chance for any. The way @Wess described would be how you get the proper odds:
<?php
$rand = rand(1, 100);
$amount = array("50", "100", "250");
if($rand <= '70') {
echo "You win $amount[0]";
} elseif ($rand > '70' && $rand < '90') {
echo "You win $amount[1]";
} else...
This is typically how applications are setup, with needed files being required through something like config.php or common.php. It's considered good practice to do this then having it in index.php (better organization, less work in the long run - especially when working with something that is...
That .htaccess file :down:
Also require() is better suited for config file, and should use foreach to get all from a dir instead of hardcoding each one. Also hipchat can be very resource exhaustive