PDO “Uncaught exception 'PDOException' .. Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll().”

you need to fetch until a row fetch attempt fails. I know you may only have one row in the result set and think one fetch is enough, but its not.

you probably have other statements where you didn't fully "fetch until a fetch failed". Yes, i see that you fetch until the fetch failed for one of the statements.

also, see closecursor()

$sql = "select * from test.a limit 1";
$stmt = $dbh->prepare($sql);
$stmt->execute(array());
$row = $stmt->fetch();
$stmt->closeCursor();

or

$sql = "select * from test.a limit 1";
$stmt = $dbh->prepare($sql);
$stmt->execute(array());
list($row) = $stmt->fetchAll(); //tricky

After struggling with this issue for days, I finally found that this worked for me:

$db = new PDO ($cnstring, $user, $pwd);
$db->setAttribute (PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, true);