<?php

$banners = array
(
	'banner1' => 1,
	'banner2' => 2,
	'banner3' => 4,
	'banner4' => 8,
	'banner5' => 16,
);

function Probability($data)
{
	if (is_array($data) === true) {
		$result = 0;
		$probability = mt_rand(1, array_sum($data));

		foreach ($data as $key => $value) {
			$result += $value;

			if ($result >= $probability) {
				return $key;
			}
		}
	}

	return false;
}

$result = array();

for ($i = 1; $i <= 100; $i++)
{
	$result[Probability($banners)]++;
}

arsort($result);

print_r($result);

?>

Array
(
    [banner5] => 41
    [banner4] => 38
    [banner3] => 10
    [banner2] => 8
    [banner1] => 3
)