Passing an sql query to a variable

I want to run a query and pass it to a variable so that I can then use that variable in another query.
$sql="select O_short_code from Office, Office_Staff where OS_O_id=O_id and OS_name='xxxxxxx'";

$short_code=CustomQuery($sql);
I then want to use the variable $short_code in another query but $short_code isn't getting a value.