разбить строку по 5 символов

function splitStringBy5Char($string) {
    $result = [];
    $currentString = '';
    for ($i = 0; $i < strlen($string); $i++) {
        $currentString .= $string[$i];
        if (strlen($currentString) >= 5) {
            $result[] = $currentString;
            $currentString = '';
        }
    }
    if ($currentString) $result[] = $currentString;
    return $result;
}

print_r(splitStringBy5Char('1234567890'));