Variable mysql statement in php

Variable mysql statement in php

I have a variable that is a filter for my query:
$filterString.=" AND venue = ".$venue;
And I want this variable (when called) to add the AND filter statement to
my query.
My query is as follows (with the failed attempt):
mysql_query("SELECT * FROM event
WHERE city = '$city' " . $filterString . "
ORDER BY date ASC");