Not for 'how-to' coding questions but PHP theory instead, this forum is here for those of us who wish to learn about design aspects of programming with PHP.
I always thought it was, and the manual seems to think so, but I got a bug report from a user who complained that DOMDocument was available. Here's their config:
This is irrelevant, look inside the /usr/share/doc/libphp5_common5-5.0.4/configure_command file. urpmi is your friend, use it to install extensions not shown below.
Well, I assume that if DOMDocument doesn't exist, then the 'dom' extension isn't loaded. True, DOMDocument doesn't show up in the extensions list, but dom should.
Anyway, I've fixed it. Surprising that you can't even count on dom being present on PHP5 installs.