How to verify that all files are intact prior to install?

Question

I'm working on my CMS (in PHP platform) for a long time now. The main program is done and I'm currently developing the Installer part. Installation itself will be fairly simple:

1. Upload all files
2. Verify that the "content/" dir has correct permissions
3. Check if ALL files are intact and not modified [This is the subject of this question]
4. Insert the config data and first settings
5. Run install (Generate all DB tables and insert sample data etc.)

Now the question-mark is at step 3. How do I verify ALL files? Verification itself should compare all CMS root-directories files against a list from remote location. List should contain filename, filesize and filetype. This way the user can check, that there are no unnecessary or corrupted files, that could indicated a breach in the software.

I have seen some software installers do that, but I cannot find any right now and there for I'm clueless on the most optimal method for this. Of course there always is a simple array trick, but there surely must be a better and faster method?!