The require() function is used in PHP to include a file in the current PHP script. Fatal error: require(): Failed opening required means that PHP could not find or open the file you specified in the require() function.

There could be several reasons for this error. Here are some possible causes:

The file path is incorrect: Make sure you have specified the correct path to the file you are trying to include.
The file does not exist. Double-check that the file exists in the specified location.
The file is not readable: Make sure that the file has the correct permissions and that the PHP process has permission to read the file.
There is a syntax error in the file: If there is a syntax error in the file you are trying to include, it will cause this error. Check the file for any syntax errors and fix them.
There is a problem with the server: If none of the above solutions work, there may be a problem with the server itself. Check the server logs for error messages that may help diagnose the issue.

To troubleshoot this error, try the following steps:

Check the file path: Make sure that the file path is correct and that the file exists in the specified location.
Check the file permissions: Ensure that the file has the correct permissions and that the PHP process can read the file.
Check for syntax errors: Open the file in a text editor and check for any syntax errors.
Check the server logs: Check the server logs for any error messages that may help diagnose the issue.

I hope this helps! Let me know if you have any other questions.