<?php

function findFirstDuplicate($arr) {
    $seen = [];
    foreach ($arr as $num) {
        if (isset($seen[$num])) {
            return $num;
        }
        $seen[$num] = true;
    }
    return null;
}

$fn = fn($arr) => findFirstDuplicate($arr);

echo $fn([2, 5, 1, 2, 3, 5]); // Output: 2