kbenton@localhost  ~  php --version    
PHP 7.2.6 (cli) (built: May 22 2018 16:22:08) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
 kbenton@localhost  ~  php testme.php 
Exception caught: SQLSTATE[HY000]: General error: 2014 Cannot execute queries while other unbuffered queries are active.  Consider using PDOStatement::fetchAll().  Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.
Here!
 kbenton@localhost  ~  cat testme.php
<?php

try {
    $pdo = new PDO('mysql:host=127.0.0.1', 'aql_app', '@ql_@pp');
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, 0);
    $pdo->query('USE aql_db');
} catch ( Exception $e ) {
    echo "Exception caught: {$e->getMessage()}\n";
}
echo "Here!\n";
 kbenton@localhost  ~ 

The relevant part here is that "USE" is not supported by prepared statements. MySQL returns error 1295 and PDO checks for that and falls back to emulated prepared statements. But presumably that doesn't work correctly.