Php To Run mysql Query In Joomla Code Behidn FIle

Hello - i need some assistance using the concat() operator in php and joomla - i have this query but it gives me error

$query = $db->getQuery(true);
$query = 'Select name, (Select Concat(first_name, ' '',last_name) FROM memburs mem WHERE mem.user_id = ac.created_by) As name1,';
$query .= '(Select Concat(first_name, " " ,last_name) FROM memburs mem WHERE mem.user_id = ac.approved_by) As name2, Date_Start, Date_End';
$query .= 'From ac where isApproved = 0';
$db->setQuery($query);
$cases = $db->loadObjectList();