How do I dynamically add features to a Layer

[Benoit Myard]

Yes, you have to use MapScript for that matter (theoretically you
could avoid MapScript but it depends much on your actual needs).

Although I can't speak for other MapScript flavors, Perl lacks
documentation and examples, hence you should avoid it in my opinion.

PHP seems to be the most widely used and extensively documented
MapScript flavor so you should start by reading its documentation [1]
and examples [2].

I'm doing a similar task at the moment and as far as I can tell, you
can't add data to an existing set of data. For example, I've tried to
add a city to a layer of cities but whenever I added it, all other
cities disappeared.

Good luck.

[1] http://mapserver.gis.umn.edu/docs/reference/phpmapscript-class/referencemanual-all-pages
[2] located in mapserver-4.10.0/mapscript/php/examples

-- 
Benoit Myard