Check for typos: Make sure that you have spelled the function name correctly and that it matches the function declaration.
Include the necessary file: If the function is defined in a separate file, ensure that you have included or required that file before calling the function.
Check for function existence: Before calling a function, you can use the function_exists() function to verify if the function is defined. This helps prevent the error from occurring.
Check for missing extensions: If the function belongs to an extension that is not enabled in your PHP configuration, you will need to enable the extension by updating your php.ini file or using the dl() function.
Reinstall or update the extension: If the function is part of an extension that is installed but still causing the error, you can try reinstalling or updating the extension.

By following these steps, you should be able to fix the "Fatal error: Call to undefined function" error in PHP.