Zen Cart Ultimate SEO URL Install Error, cannot redeclare zen_reset_cache_data_seo_urls()

Earlier I was working on a Zen Cart site and ran into an issue installing the Ultimate SEO URL add-on. The Ultimate SEO URL add-on creates more SEO(Search Engine Optimization) friendly URL’s for Zen Cart. The site in question already had a previous version of the Ultimate SEO URL add-on installed and once I was finished the front-end of the site was working perfectly with the new SEO friendly URL’s. The problem was when I attempted to login to the admin portion of the Zen Cart site I was receiving a “500 – Internal Server Error” message. Below I have noted information on what was done to troubleshoot the problem as well as how the problem was resolved.

500 Internal Server Error When Opening Zen Cart Admin:

Troubleshoot Problem Installing Zen Cart Add-On Ultimate SEO URL:

The first thing I attempted to do was change all of the files uploaded with the Ultimate SEO URL Zen Cart add-on back to their original state to make sure this resolved the issue. The problem was even after setting everything back to the default settings the 500 error still existed so I was starting to get frustrated at this point thinking that the owner of the site was also making configuration changes at the same time.

After still getting the error after setting everything back to the way it was when I started I next installed the Debug Error Logging Utility add-on so I could obtain more details about what was going on. Sure enough the problem actually with the Ultimate SEO URL add-on as noted in the output of the Debug Error Logging Utility below.

Zen Cart Debug Error Logging Utility Output:

I tried to comment out the function that was causing the problem but I was still unable to login to the Zen Cart admin for this site. I again looked at the output from the Debug Error Logging Utility to see if it would point me in the right direction.

Second Zen Cart Debug Error Logging Utility Output:

After seeing the above error which was very similar to the first I figured out the problem. The issue was that I had simply renamed the old seo.php file in the /admin/includes/functions/extra_functions/ directory to .OLD instead of deleting it or replacing it. It must be that in the extra_functions directory all files are processed regardless of the ending. Once I deleted the .OLD file and uploaded all of the other Ultimate SEO URL add-on files again the Zen Cart admin worked without issue.