Using CMS made simple (1.1.11 and 1.1.12), I hit problems with php files that I was trying to "include" in a User-Defined Tag (UDT). When I tried to save a Template, I hit a blank page (or occasionally, pages complaining about File Not Found) rather than returning to the Admin pages.
Eventual solution:
Do NOT use cwd() etc for capturing relative paths - "cms made simple" (cmsms) redirects pages, so that you send it off to find a file with an address that ends \pages\phpfiles\myfile.php (where "pages" is the prime cmsms directory) you will hit complaints that it cannot find the file \pages\phpfiles\admin\myfile.php
So if you need the UDT to "include" file a.php, and a.php pulls in b.php, then have a.php specify the path from there to the file b.php (if in the same directory, simply "include (b.php);'
And what path to the first file? Use your HOST's path
eg for my host
include("/home/users/[USER NAME]/html/[DOMAIN NAME]/[LOCAL FILE STRUCTURE]/thisFile.php");