Open_basedir preventing fopen(...,"w")

I get the following when trying to write to a temp file:

I don’t understand, since /tmp is in the list of permitted directories. I’ve also tried to write to a directory in /home/wtlladm, also in the path, with the same error.

What do I have to do to open a file to write?

Hi, this article from the KBase might help:

PHP Security restrictions