PHP Array: <select> box and <optgroup>

Question

I'm trying to code a script that allows you to generate a with the cities of each country. Cities should be grouped by country through PHP array.

So this example here, in HTML, it is what I wanted but in PHP Array.

<select>
    <optgroup label="Country X">
        <option value="City1">City 1</option>
        <option value="City2">City 2</option>
        <option value="City3">City 3</option>
        <option value="City4">City 4</option>
        <option value="City5">City 5</option>
    </optgroup>

    <optgroup label="Country Y">
        <option value="City6">City 6</option>
        <option value="City7">City 7</option>
        <option value="City8">City 8</option>
        <option value="City9">City 9</option>
        <option value="City10">City 10</option>
    </optgroup>

    <optgroup label="Country Z">
        <option value="City11">City 11</option>
        <option value="City12">City 12</option>
        <option value="City13">City 13</option>
        <option value="City14">City 14</option>
        <option value="City15">City 15</option>
    </optgroup>
</select>

Here's the code what I've done so far with PHP... The main problem of this one is each country has all the cities. I only wanted 5 cities each in different countries.

<?php

$countries = array(
    'Country X',
    'Country Y',
    'Country Z', 
);

$cities = array(
    'City 1',
    'City 2',
    'City 3', 
    'City 4',
    'City 5', 
    'City 6',
    'City 7',
    'City 8', 
    'City 9',
    'City 10', 
    'City 11',
    'City 12',
    'City 13', 
    'City 14',
    'City 15', 
);


?>

<select name="places">
<?php foreach($countries as $key): ?>
        <optgroup label="<?php echo $key; ?>">
        <?php foreach($cities as $key => $value): ?>
            <?php echo '<option value="'.$key.'">'.$value.'</option>'; ?>
            <?php endforeach; ?>
        </optgroup>
<?php endforeach; ?>
</select>

Answer 1

Split cities in chunks:

$cities = array_chunk($cities, 5);
$counter = 0;?>

<select name="places">
<?php foreach($countries as $key): ?>
        <optgroup label="<?php echo $key; ?>">
        <?php foreach($cities[$counter++] as $key => $value): ?>
            <?php echo '<option value="'.$key.'">'.$value.'</option>'; ?>
            <?php endforeach; ?>
        </optgroup>
<?php endforeach; ?>
</select>