How do I compare php files

Is there an editor (or other way) that will compare two php files with each other.

Example:

I take a clean version of index.php

I then make changes to index.php for my theme – for the purpose of this topic call it index2.php

Is there an editor (or other way) to open both files at the same time ( index.php and index2.php) and then compare the contents of one file with the other to see where any changes were made

For example:

changes made in the index2.php file – compared to the original index.php

Hopefully, I explained this properly

Thanks