combine multiple .tif into one with irfanview/php

I need to take multiple .tif files with (close to) the same name, and combine them using irfanview into one file. I can get the file to test (which seems to be testing correctly) by commenting out some lines, but when I actually execute the file, it just hangs. Do you see any potential problems with this code?

http://paste.ee/p/pVBFu