Chances.php

<?php

class Chances {
	
	protected $max;
	protected $pie;
	protected $rnd;
	
	public function __construct($max = 100) {
		$this->max = $max;
		$this->pie = $max;
		$this->rnd = rand(1, $max);
	}
	
	public function percent($i) {
		$this->pie -= $i;
		
		return $this->rnd <= ($this->max - $this->pie);
	}
}


// Test

$count = [];

for ($i=0; $i < 10100; $i++) {
	$chances = new Chances();
		
	if ($chances->percent(15)) {
		$count[15] = isset($count[15]) ? $count[15] + 1 : 1;
	} elseif ($chances->percent(30)) {
		$count[30] = isset($count[30]) ? $count[30] + 1 : 1;
	} elseif ($chances->percent(55)) {
		$count[55] = isset($count[55]) ? $count[55] + 1 : 1;
	} else {
		// fail
		$count[0] = isset($count[0]) ? $count[0] + 1 : 1;
	}
}

print_r($count);
/* Output
(
    [30] => 3057
    [55] => 5533
    [15] => 1510
)
*/