PHP/MySQL - Multiple queries at the same time

Question

I have 24 databases with a table labeled email_queue.

I have another database with a list of all the databases that have the email_queue table

Answer 1

I think having this many databases is probably a sign of bad design. If you can't change it and need to move forward now, I suggest one of two options:

1. Run the same script with a parameter to select which database to use. You should be able to find resources on how to do this
2. Use non-blocking queries; the rest of this answer will be spent talking about this.

Here's a somewhat complete example using the mysqli extension (requires the mysqlnd driver):

$credentials = array(
    array(
        'host' => 'host1',
        'user' => 'user',
        'password' => 'password',
        'database' => 'database'
    ),
    array(
        'host' => 'host2',
        'user' => 'user',
        'password' => 'password',
        'database' => 'database'
    ),
    // credentials for other sites
);
$dbcs = array();
foreach ($credentials as $config) {
    $dbcs[] = array($db = new mysqli(
        $config['host'],
        $config['user'],
        $config['pass'],
        $config['database']
    ));
    $query = ""; // here is your query to do whatever it is with your table
    $db->query($query, MYSQLI_ASYNC);
}

$results = array();
$errors = array();
$rejected = array();
$secondsToWait = 1;

while (!empty($dbcs)) {
    foreach ($dbcs as $key => $c) {
        $db = $c[0];
        if (mysqli_poll($c, $errors, $rejected, $secondsToWait) == 1) {
            $r = $db->reap_async_query();

            // here you would do your fetches for each query, such as
            $results[] = $r->fetch_assoc();

            // do what you need to do with the result

            // then cleanup
            $r->free();
            $db->close();
            unset($dbcs[$key]);
        }
    }
}

Note that it does have drawbacks, such as a failed query may bring down the whole program.