MySQL 5.5 SIGNAL and PDO exceptions - php

Question

I have a TRIGGER for a MySQL table which raises exception with SIGNAL when there's something wrong with the data provided. How can I catch this exception in PHP with PDO?
Thanks in advance.
UPDATE
More information on my problem: I am executing several queries within a transaction and I want to rollback the transaction if one of them fails because there was a SIGNAL. Currently a signal is activated but all the other queries are executed.
ANOTHER UPDATE
So far I've got:
try {
$db->beginTransaction();
if (!$db->query('UPDATE accounts SET amount = amount - 10 WHERE id = 1')) {
throw new Exception($db->errorInfo()[2]);
}
if (!$db->query('UPDATE accounts SET amount = amount + 10 WHERE id = 2')) {
throw new Exception($db->errorInfo()[2]);
}
$db->commit();
} catch (Exception $e) {
$db->rollback();
echo 'There was an error: ' . $e->getMessage();
}
Is this the proper way to do a transaction with handling the exceptions?
Sorry if my question is too broad.

Answer 1

Try something like:
$ls_error = "";
$mysqli->autocommit(FALSE);
// first sql
if (!$mysqli->query($sql)) {
$ls_error .= sprintf("Error: %d: %s\n",$mysqli->errno, $mysqli->error);
$mysqli->rollback();
}
// second one
if ($ls_error == "") {
if (!$mysqli->query($sql)) {
$ls_error .= sprintf("Error: %d: %s\n",$mysqli->errno, $mysqli->error);
$mysqli->rollback();
}
}
// third one
if ($ls_error == "") {
if (!$mysqli->query($sql)) {
$ls_error .= sprintf("Error: %d: %s\n",$mysqli->errno, $mysqli->error);
$mysqli->rollback();
}
}
if ($ls_error == "") {
$mysqli->commit();
} else {
echo $ls_error;
}

Answer 2

Abstract example. I have the solution works without any problems.
Trigger:
IF NEW.value < 0 THEN
SET #msg = CONCAT('Wrong count_from value! ', NEW.value);
SIGNAL SQLSTATE 'HY000' SET MESSAGE_TEXT = #msg;
END IF;
php (PDO):
try {
$conn->query('INSERT INTO `table` SET `value` = -1');
} catch (Exception $err) {
$error_message = $conn->query('SELECT #msg')->fetchColumn();
}