Description:
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When you use PDO::beginTransaction(), do a query and issue PDO::commit(), the result of the last call is bogus at best. 

Documentation states "Returns TRUE on success or FALSE on failure.". In current implementation, this is wrong. In fact, it returns TRUE in all cases I tested - even for _impossible_ commits. The only time it actually fails is when issuing a PDO::commit() while no transaction is active. This will, regardless of what you set PDO::ATTR_ERRMODE to, result in a PDO::EXCEPTION (that is another bug, see #63546).

Unfortunately, there is another bug in the implementation of PDO::commit() that makes it almost impossible to detect that error since "MySQL server has gone away" or "Error reading result set's header" are only issued as as warnings and neither PDO::errorCode() nor PDO::errorInfo() are set, nor is a PDOException thrown. PDO_ATTR_ERRMODE is also not beeing honored. I filed that bug under #66528 and will only focus on the bug described above in this report.

The easiest way to reproduce is to start a transaction, issue a query, sleep for a couple of seconds and while doing that, shutting down the MySQL-Server. When the server is clearly gone, issue PDO::commit(). Although PDO knows it can under no circumstances have commited the data, it returns success for the operation.

Test script:
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<?php
$dbh = new PDO('mysql:dbname=test;host=127.0.0.1;charset=UTF8', 'testuser', '');
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_SILENT);

// prepare table for test
$dbh->query('DROP TABLE IF EXISTS importantdata');
$dbh->query('create table test.importantdata (a int) engine=innodb');

$dbh->beginTransaction();
$dbh->query('insert into importantdata (a) VALUES (1), (2)');

sleep(20); // shut down mysql-server

var_dump($dbh->commit());

?>

Expected result:
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bool(false)

Actual result:
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bool(true)