How to fix: undefined is not a function?

How to fix this this error Fatal error: Call to undefined function...?

• Fatal error: Call to undefined function wp() in /home/content/33/7211533/html/someSite/wp-blog-header.php on line 14 can anyone figure whats the problem? I already update the wp-includes and wp-admin folder, my functions.php is fine but havent figure how to fix it any good suggestions? ----EDIT---- hi guys Im back, theres only 2 plugin installed its the http://wordpress.org/extend/plugins/wp-db-backup/ I cant really figure out on what happen on the website, but what I did to fix this weird problem ,...is I reinstall wordpress , (upload the new copy of wordpress on the site, and I make sure its latest one) - equivalent to a http://codex.wordpress.org/Updating_WordPress#Manual_Update. After that its started to work fine... so this problem is now fixed...

• Answer:

  wp() is quite essential function. It resides in functions.php file (don't confuse with functions.php in theme, different thing) and I don't think you can load WP without passing require directive for that file at some point. So your install seems very broken - either functions.php file in core is damaged or something else is broken and makes it skip loading that file.