The first thing to diagnose is what is actually in the database (use PHPMyAdmin or CLI).
You will of course either find that the backslash is either in the database or not.
If the backslash is in the database you probably have magic quotes gpc/runtime on and/or are calling the "addslashes()" function which does this.
If you are escaping your data with mysql_real_escape_string() then think again, you probably have magic quotes gpc enabled either in php.ini or........
I couldn't figure out why this wouldn't work, a test script in the root of my htdocs folder worked fine.
Within some subdirectories the same code would produce different base64 results but I didn't know hwy.
Zip file size: 6888 bytes, number of entries: 92
error [/tmp/archive.zip]: missing 242827681 bytes in zipfile
(attempting to process anyway)
error [/tmp/archive.zip]: attempt........