SoftUni Parking

<?php

$n = intval(readline());

$parking = [];

for ($i = 0; $i < $n; $i++) {
    $parkingLot = explode(' ', readline());
    $username = $parkingLot[1];

    if ($parkingLot[0] === 'register') {
        $carNumber = $parkingLot[2];
        if (key_exists($username, $parking)) {
            echo "ERROR: already registered with plate number " . implode('', $parking[$username]) . PHP_EOL;
        } else {
            $parking[$username][] = $carNumber;
            echo "$username registered $carNumber successfully" . PHP_EOL;
        }
    } elseif ($parkingLot[0] === 'unregister') {
        if (!key_exists($username, $parking)) {
            echo "ERROR: user $username not found" . PHP_EOL;
        } else {
            echo "$username unregistered successfully" . PHP_EOL;
            unset($parking[$username]);
        }
    }
}

foreach ($parking as $user => $carRegistration) {
    echo "$user => " . implode('', $carRegistration) . PHP_EOL;
}