Fatal error: Uncaught Error: Call to undefined function

When your WordPress site can not be display, you need to view the error in order to fix it. Then you need enable error to be display and enable wordpress debugging, and you will see “Fatal error: Uncaught Error: Call to undefined function”. The “Call to Undefined Function” is a result of one of the below cases, and you need to choose the one that you think caused the problem, depending on your situation.

Do you need help? If yes, then you should read this blog.

Error with WordPress plugin or theme

This is the first thing that can cause problems, read carefully and try to get info from the error which could have mentioned which plugin or theme causing it.

if you’ve installed a theme or a plugin, without verifying the compatibility with the WordPress versions, then, you need to check the details again. With thousands of WordPress plugins, people install them fast, and they only notice the rating, the name, but not, the plugin compatibility with its version.

You can also deactivate the plugin or theme one by one and see if the problem was solved or not, that will help as well.

Error with php extension

This is the second thing to verify. If read carefully and notice that a php extension missing. Then you need to install it in order to solve it.

For example mb_strtoupper() function is the part of mbstring extension of PHP. By default “mbstring” extension is not available to php. We have to install mbstring extension manually.

Error when upgrade the php version

This is the third thing to verify. If you’ve upgraded your php version to the latest 7.4 or 8.1, from your hosting control panel, and get the error, then, make sure to change back the php version manually.

The reasons why latest php version had error due to the plugin or theme could not support it which is incompatible with it.

WordPress multisite plugin on a single site

This is another mistake that can cause undefined function errors. Make sure that you are using a plugin for your installation type. The multisite plugins won’t work on a single site installation in the majority of cases, and vice versa.

In this case, you need to verify the plugin details and info page and look if you can use it on your single site or not. At the same time, make sure to update your WordPress to the latest version. That will solve the problem if it was in the version compatibility.

Not an existing function

This happens to developers in general, but if you’ve deleted a plugin or a theme, or even a file, then, you will notice errors about missing functions. This case needs lots of debugging and investigation in the WordPress files.

So, if you get errors, just read carefully the exact message that you get, and follow its destination or the error that you get. Then, look at the declared destination or function, and see if you’ve errors there. Check your “functions.php” file and see if you have the missed function or not. Then, add it manually or ask someone who knows exactly how to do that.

Please do contact us if above steps failed.