While updating WordPress 3.6.1 on my site I am getting the following error and the site crashes. Why is this happening to my site, and how can I overcome this problem?
Following error arises:
Fatal error: Call to undefined function wp_initial_constants() in /home/adventec/public_html/wp-settings.php on line 26
Try re-uploading all files & folders - except the wp-content folder - from a fresh download of WordPress.
Make sure that you delete the old copies of files & folder before uploading the new ones.
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.