Description:
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this bug was found accidently using the wrong pdo command:
exec instead of query!

the use of exec crashes the apache.
an errormessage should be given, reporting of the wrong command use at this position. 

Reproduce code:
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$table = "SELECT session_id FROM ". DB_PREFIX ."session ";
$where = "WHERE session_id='$session'";

FALSE:
$session_id = $Db->exec($table.$where);

RIGHT:
$session_id = $Db->query($table.$where);

Expected result:
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throw out errormessage!

Actual result:
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crash of apache!