FixRunner.com explains how to handle a WordPress undefined function fatal error.

What’s the problem?

  1. You have just updated WordPress
  2. You try to log in to your WordPress admin
  3. You receive the following WordPress undefined function is_network_admin() error message: “Fatal error undefined function is_network_admin()

How to solve WordPress undefined function error

A manual update is oftentimes the best solution for this type of error. Very often, a WordPress undefined function error occurs as a result of a glitch during WordPress update, which a manual update would usually fix.

Follow these steps to update your WordPress site manually:

Step 1

Download and unzip the latest wordpress file.
Download and unzip the - Fix WordPress undefined function error

Step 2

Be sure to back-up your WordPress and all files

Step 3

Next, you need to rename your wp

Step 4

Now you need to upload the wp-includesand wp-admin directories. Take them from the unzipped folder on your desktop and upload them to your web host via FTP
upload the wp-includes and wp-admin directories - Fix WordPress undefined function error

Step 5

The next step is to upload your files from the new wp-content folder to your old wp-content folder.
upload your files from the new wp-content - Fix WordPress undefined function error

Step 6

Replace and overwrite the existing files.
Replace and overwrite the existing files - Fix WordPress undefined function error

Step 7

Go into your WordPress root directory and upload the new version of the remaining files

Step 8

Still in your WordPress root directory via FTP, remove .maintenance
Remove .maintenance - Fix WordPress undefined function error

Step 9

Log into your WordPress admin as usual and follow the link that you are given. Follow the instructions that appear on screen.

Step 10

lf you have enabled caching, clear your cache.
 
This should take care of the error and you should now be able to use your site.
If you however still encounter this problem, it may be time to have a pro look at it. Request for our service and one of our WordPress experts will get to work fixing your site right away.
 
Also, if you have been able to solve this issue using other steps, kindly share these with us and our readers.