$users_in = (\IPS\Db::i()->select('*', 'nexus_invoices', 'i_member',$member_id ));
foreach ($users_in as $user_in) {
	//example usage of it
	//echo $user_in['ps_id']
}
	also, using first-> without checking it will give internal server error. If you only want to get first query the way you should use is:
 
$user_in = (\IPS\Db::i()->select('*', 'nexus_invoices', 'i_member',$member_id ));
if (count($user_in) > 0) {
	$user_in = $user_in->first();
}
	 
 
	Also, forgot to mention, you should put your where statement in array.
 
array('i_member=?', $member_id')