Too bad you modified the file ... :-/
The OS (at least on Unix), won't really keep track of history, except for some "last modified" info.
On the other hand, look into:
1- file permission and ownership. If your user is the only one allowed to change it (and you're
100% sure that you did not add this line) , then your account might be compromised.
2- Backup/history of your file. Retracing your history log will show you at what stage the line was added. You did use version control, right?
3- Some Unix file systems have journaling functions. Depending on the one you're using, it might be possible to look into thid.
4- Telling us 'base 64 garbage' is not really helpful. If you could reconstitute the original text it could help us reconstruct what happened.
By the way you have an unmatched quote (
obligatory xkcd reference).
Also; you're going to have to tell us more about the file.
Is it written by you or generated?
What text editor/IDE did you use?
How are you executing the PHP script?
What OS is underlying all this.
...
Long story short, if you want a real answer, we're going to need more, much more info than that.