<?php

$twoHeads = 0;
$total = 0;
for($i = 0; $i < 1000000; ++$i) {
    $coinOneHeads = mt_rand(0, 1);
    $coinTwoHeads = mt_rand(0, 1);

    if( !$coinOneHeads && !$coinTwoHeads ) {
        continue;
    }

    ++$total;
    if( $coinOneHeads && $coinTwoHeads ) {
        ++$twoHeads;
    }
}

echo $twoHeads / $total;