PDO MySQL transaction not rolling back properly or not rolling back at all

Hi,

I have the following code that runs a bulk insert statement, I have it all wrapped in  a transaction but when the insert causes an error, I get left with records in the database, which there shouldn't be because I rollback the transaction. So I am not sure if it is because I am running the insert as a prepared statement or if something in my code is just off and I can't see it.

 //next run additions
    unset($values);
    if($additions_count > 0)
    {
    	   $conn = new connection();  	    
	   $conn->openPDODB();
	   $dbh->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
	   
	    for($a = 0;$a<$additions_count;$a++)
	    {
	    	    if($additions[$a]->get_password() == ""){$additions[$a]->set_password(gen_password());}
	    	    $values[] = '("' . $additions[$a]->get_username() . '","' . $additions[$a]->get_password() . '","' . $additions[$a]->get_salt() . '",' . $additions[$a]->get_licensing_entity_license_id() . ',"' . date("Y-m-d h:m:s") . '",' . $additions[$a]->get_pwd_change_required() . ')';
	    }
	    
	    $strSQL = "Insert into `" . STUDENT_TABLE . "` (`username`,`password`,`salt`,`licensing_entity_license_id`,`date_created`,`require_password_change`) Values " . implode(',',$values);
	    $dbh->beginTransaction();
	    $stmt = $dbh->prepare($strSQL);
	    try
	    {
	    	    $stmt->execute();
	    }
	    catch (PDOException $e)
	    {
	    	    $dbh->rollBack();
	    	    $ec = $stmt->errorCode();   	    	    
	    	    cleanup_and_exit("Location:/eas/upload_student_file.php?err=$ec",$handle);
	    	    //$insert_exceptions[] = $additions[$a];
	    }
	    $dbh->commit();
	    $values = array();   
    }

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I have a constraint on the table that username/password combination is unique. When I upload a file that violates that constraint in one of the rows, it handles the error and reports back the correct error, but I end up with a partial insert into the database that remains instead of being rolled back.