subquery does not work on php

subquery does not work on php

I have table. I need in my query count some field. This is my query:
select COUNT(q1) as 'result'
from (select DISTINCT id_qs as 'q1' FROM main where id_exam=40) as q2
It's query perfect work. But. When I put it in php code:
function HowManyQuestion($id)
{
mysql_connect($this->hostname,$this->username,$this->password) OR
DIE("Can't connect");
mysql_select_db($this->dbName) or die(mysql_error());
$query = "select COUNT(q1) as 'result' from (select DISTINCT id_qs
as 'q1' FROM main where id_exam=40) as q2;";
$res = mysql_query($query) or die(mysql_error());
$id=0;
while ($row=mysql_fetch_array($res)) {
$id=$row['result'];
break;
}
return $id;
}
In result I have error:
You have an error in your SQL syntax; check the manual that corresponds to
your MySQL server version for the right syntax to use near ') as q2' at
line 1
So how this fix?