What is admin.php?

I’m seeing errors appear in my error_log file that refer to the file “admin.php”, here is an example:

[20-Apr-2015 21:45:13 Europe/London] PHP Fatal error:  Incompatible file format:  The encoded file has format major ID 1, whereas the Loader expects 5 in /mypath/public_html/admin.php on line 0

Firstly, what does this mean? Also what is the admin.php file and do I need it?
To help answer, here is some background on the site. When I first took over control of the site, a few years ago, it used some kind of Frontpage related CMS that was terrible and I did not like (it only worked right in IE, not in good browsers, wierd!). I was at the time learning html and css, so thought I would have a go at building a hand-made html site form scratch. I did this and replaced the CMS with my hand-made site and deleted as much as I dare of the old system. At the time I was just getting my head around html and css, I didn’t at the time really know waht php was. I have since then learnt about php and re-built the site using php.
Now, this admin.php file, I’m thinking its just some throw-back to the old CMS and is no longer needed (it is last modified in 2010, which makes me think this). On the other hand it may be some important system file that I should not delete. If I open the file, it is not in any format I can read, just symbols and nonsense (to my eye). The fact that it is cropping up errors now suggests someone or something it still trying to use the file.
Can anyone give me a clue?

That sounds like it’s binary instead of text.

Are you using encrypted files?


Is it possible the last time you uploaded it your FTP settings were binary instead of text?

If your backup copy doesn’t look like “symbols and nonsense” try replacing the borked file with your backup copy.

I just found an old back up of the site, from when it used the CMS. The admin.php file is there and it looks exactly the same as the current one.
I was wondering if I actually need the file, is it an important one or obselete?
Since I have a back-up, I could try removing it and see what happens.

Since the error is referring to that file the code obviously needs it. The error message refers to it being an encoded file - presumably that is to prevent people reading/editing the content.

While your backup copy may look similar it may not be completely the same if the current live copy has been corrupted.

I just uploaded to the old back up and replaced the one on the server. It does not appaer to make any difference. The site still functions as expected. If I type in admin.php to the address to check it, I get a blank screen and another error appears in the log.

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