[laimas]

$a = [
    1234576,
    2456901,
    1234598,
    2456776,
    2456993,
    1234567,
    1234555,
    3245089
];

$len = 4;

for($i = 0, $j = count($a)-1, $k = ceil(count($a)/2); $i < $k; $i++, $j--) {
    $group[substr($a[$i], 0, $len)][] = $a[$i];
    $group[substr($a[$j], 0, $len)][] = $a[$j];  
}

print_r($group);

print_r(array_values($group));