Inefficiency of mt_reand() in PHP

<?php
////
function flip($h, $t, $times) {
    if ($h + $t <= $times) {
        $a = mt_rand(0, 1);
        if ($a == 1) {
            return flip($h + 1, $t, $times);
        } else {
            return flip($h, $t + 1, $times);
        }
    } else {
        return $h . ',' . $t;
    }
}
/////
function trial($l) {
    $ar = array(
        'null'
    );
    for ($i = 1; $i <= $l; $i++) {
        array_push($ar, flip(0, 0, 29));
    }
    $countAr = array_count_values($ar);
    arsort($countAr);
    foreach ($countAr as $key => $value) {
        print "<b>$key</b>" . ": " . $value . "</br>";
    }
}
trail(500);
?>


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