Wordpress - Fatal error: Call to undefined function wp() in ../wp-blog-header.php?

Your wp-config.php file might be empty for some reason.

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.