Mandriva 2007 & maybe other distros have this issue
I use mandriva and ran into this crap issue....
Mandriva likes to make modules of _everything_ for apache and I was getting the same errors and ISPConfig was not loading in new websites because httpd -t was throwing a vail but invalid error.
The problem begins when you instaall a apache2 module and it installs it into 'modules.d/70_mod_php4.conf' or something like that.
httpd -t doesn't load that in -- know why ? Cause in Mandriva atleast 1 issue is going on.
1) httpd -t doesn't check syntax with the included 'Include modules.d/*.conf' still
So what I had to do was manually add the following into httpd.conf ....
LoadModule php4_module extramodules/mod_php4.so
LoadModule suexec_module extramodules/mod_suexec.so
These two modules _MUST_ be loaded into the main httpd.conf for 'httpd -t' to work. After I did that the error 'Invalid command 'php_admin_flag' went bye bye and all was good.
The suexec_module will also throw a error if its loaded the same way the php4 module is so I had to add that as well.
Maybe 'httpd -t' just isn't a good idea until the apache people fix this bug. I already reported it.