Percentage chance based on a roll of rand()

roll.php

$roll = rand(0,99);
$percentage_chance = 10;

// Override based on IP for testing
if (in_array($_SERVER['REMOTE_ADDR'], array('127.0.0.1'))) {
    $percentage_chance = 100;
}

if ($roll < $percentage_chance) {
    echo 'Within 10% roll chance';
} else {
    echo 'Outside of roll chance';
}