PDO prepare silently fails

Question

I'm experimenting with PHP's session_set_save_handler and I'd like to use a PDO connection to store session data.

I have this function as a callback for write actions:

function _write($id, $data) {
    logger('_WRITE ' . $id . ' ' . $data);
    try {
        $access = time();
        $sql = 'REPLACE INTO sessions SET id=:id, access=:access, data=:data';
        logger('This is the last line in this function that appears in the log.');
        $stmt = $GLOBALS['db']->prepare($sql);
        logger('This never gets logged! :(');
        $stmt->bindParam(':id', $id, PDO::PARAM_STR);
        $stmt->bindParam(':access', $access, PDO::PARAM_INT);
        $stmt->bindParam(':data', $data, PDO::PARAM_STR);
        $stmt->execute();
        $stmt->closeCursor();
        return true;
    } catch (PDOException $e) {
        logger('This is never executed.');
        logger($e->getTraceAsString());
    }
}

The first two log messages always show up, but the third one right after $stmt = $GLOBALS['db']->prepare($sql) never makes it to the log file and there's no trace of an exception either.

The sessions db table remains empty.

The log message from the _close callback is always present.

Here's how I connect to the database:

$db = new PDO('mysql:host=' . DBHOST . ';dbname=' . DBNAME, DBUSER, DBPASS);
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

I have PHP 5.2.10.

I tried to simply run $GLOBALS['db']->exec($sql) with a "manually prepared" $sql content, but it still failed silently. The query itself is all right I was able to execute it via the db console.

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Edit:

After VolkerK has identified the problem I've found this article which explains the reason behind this weird phenomena. Perhaps it could be informative to others as well.

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2nd edit:

The least painful, magic solution is that I had to add the below function call to the very end of my front controller (the main index.php) file:

session_write_close();

Answer 1

My bets are on: $GLOBALS['db'] is not set or not an instance of pdo (anymore?) and therefor a PHP Fatal error: Call to a member function prepare() on a non-object occurs and php bails out.

$sql = 'REPLACE INTO sessions SET id=:id, access=:access, data=:data';
logger('This is the last line in this function that appears in the log.');
if ( !isset($GLOBALS['db']) ) {
  logger('there is no globals[db]');
  return;
}
else if ( !is_object($GLOBALS['db']) ) {
  logger('globals[db] is not an object');
  return;
}
else if ( !($GLOBALS['db'] instanceof PDO) ) {
  logger('globals[db] is not a PDO object');
  return;
}
else {
  logger('globals[db] seems ok');
}

$stmt = $GLOBALS['db']->prepare($sql);
logger('This never gets logged! :(');

Answer 2

Perhaps PDO doesn't recognize the REPLACE INTO syntax. If the underlying DB access library doesn't support prepared statements directly, PDO emulates them, and may not have REPLACE INTO in its list of possible statement types.

Try checking $stmt->errorCode() immediately after the prepare call?

If this is mysql, you could try rewriting the prepared statement as follows:

INSERT INTO sessions (id, access, data)
VALUES(:id, :access, :data)
ON DUPLICATE KEY UDPATE
    access=VALUES(access), data=VALUES(data);

and see if that gets you any farther.