PHP error- Fatal error: Call to undefined function xxx

The error Fatal error: Call to undefined function xxx occurs when a function is called while it has not yet been created. This can also be the reason that the path to the function file is not correct.
You can therefore check that you have not made any mistakes in the path to your function, in addition you can check for any typing errors that may have been made. Depending on your text editor, you can often find out where any error is.
Also the cause may be that the function is called incorrectly, for example when a condition is not yet true.

For more information you can look at the site of PHP. https://php.net/