MySQL UPDATE query

[grainne diver]

I'm kinda new at this so if anybody could help me out i'd really appreciate it.
Ok basically i've got this, three tables
1. maps
2. posts
3. posts_text
I want to update a field "post_severity" in my "maps" table based on its "forum id" where "forum_id" is equal to the "forum_id" from "posts" table,forum_id is based on "post_id", where "post_id" from the "posts" table is equal to "post_id" from the "posts_text" table.

This is what i've come up with so far

"UPDATE maps SET post_severity = '$post_severity' where forum_id = (SELECT posts.forum_id FROM posts_text, maps, posts WHERE posts.post_id = $post_id and posts_text.post_id = $post_id and phpbb_posts.forum_id = maps.forum_id)";

I get an error saying there is an error with my sql syntax near "=(SELECT ...)