PHP File rename encode and decode

I need a small PHP function for changing the file name with encode function, I can decode the file name back to normal for checking. now I have 1000 files, so I need a loop function for all these file rename function. You can make a simple of 10 records.

Example file:
4070000667862.pdf > base64_encode > NDA3MDAwMDY2Nzg2Mg==.pdf
4070000667863.pdf > base64_encode > NDA3MDAwMDY2Nzg2Mw==.pdf
4070000667864.pdf > base64_encode > NDA3MDAwMDY2Nzg2NA==.pdf

Filename need to be rename under the folder, I am using Windows

This task need to be done within in 3 hours.