a.php

<?php

include('../config.php');

$query = "
    SELECT
        id,
        pseudo
    FROM users
";

try
{
    // These two statements run the query against your database table.
    $stmt = $db->prepare($query);
    $stmt->execute();
}
catch(PDOException $ex)
{
    // Note: On a production website, you should not output $ex->getMessage().
    // It may provide an attacker with helpful information about your code.
    die("Failed to run query: " . $ex->getMessage());
}

// Finally, we can retrieve all of the found rows into an array using fetchAll
$info = $stmt->fetchAll();
$id = $array[0]['id'];
print_r($info);



switch ($_REQUEST['action']) {
  case 'sendMessage':
    $query = $db->prepare("INSERT INTO messages SET id_user = ?, content=?");
    $query->execute([$id,$_REQUEST['message']]);
    break;
}
 ?>