I recently received a note from a customer regarding a problem getting one of the provided scripts working on a domain hosting server at work.  I looked into that problem, and fixed it- but in the process noted that the form mail and guestbook scripts I had written for the server were no longer functional.  Since the error message indicated some issue with the open_basedir option (set on a per-vhost basis via the Apache directive php_admin_value so as to not disrupt some internal services), I tried changing it for the test vhost set up on the system.

The two ways I could get the two scripts working were:

1) Remove the open_basedir restriction.  This is how the system was set up initially, but was soon restricted for security reasons.  This is not a practical long-term solution.
2) Move the scripts (the real filesystem location!) to a subdirectory of the directory containing all vhosts.  This is what I have active now, in order to keep 40-odd websites functioning.

I tried using a symlink in the test domain's webroot, but that failed as well.

I can understand this affecting include()/fopen() and similar "direct" access to files;  I don't understand why this affects HTTP POST operations.