PHP Odd Occurrences In Array
function oddOccurrencesInArray(array $a): ?int
{
    $count = [];
    foreach ($a as $value) {
        if ( ! isset($count[$value])) {
            $count[$value] = 1;
        } else {
            unset($count[$value]);
        }
    }

    return key($count);

}

This uses XOR to cancel out pairs, leaving only the value that appears an odd number of times.