It seems like that you are using nano to view the logs which is cutting out the line at the end of your screen.
What you need to do is use the
cat command for example to see the full line. So what you could do is the following:
cat /path/to/error_log | grep 'PHP message: PHP Fatal error'
Note: change the
/path/to/error_log part with the actual path to your error log.
This would output all of the lines in your error log that contain the
PHP message: PHP Fatal error message. Then you would be able to see in which file exactly is this undefined function called at.
Hope that this helps.