$numCouples = 3000;
$girls = 0;
$boys = 0;

for($i = 0; $i < $numCouples; $i++) {
    haveFamily();
}
echo "$girls / $boys\n";
echo $boys / ($girls + $boys);

function haveFamily() {
    global $girls,$boys;
    $done = false;
    $myGirls = 0;
    $myBoys = 0;
    while(!$done) {
        $rand = mt_rand(0,  getrandmax());
        if($rand % 2 == 0) {
            ++$myGirls;
            $done = true;
        } else {
            ++$myBoys;
        }
    }
    $girls += $myGirls;
    $boys += $myBoys;
}