gistfile1.aw

<?php
$m = mt_rand(1, 100);
$list = range(1, 100);
unset($list[array_search($m, $list)]);

function missing(array $list) {
    $n = count($list);
    $allSum = ($n+1)*($n+2)/2;
    return $allSum - array_sum($list);
}

echo missing($list) . "\n";
echo $m . "\n";