count.php

<?php

$a = [1, 1, 1, 2, 3, 3, 4, 5, 5, 5, 5, 6, 6, 6, 6];
$count = [];
foreach ($a as $v) {
    if (isset($count[$v])) {
        unset($count[$v]);
    } else {
        $count[$v] = true;
    }
}

echo join(' ', array_keys($count));