I would like to install some form of intranet system to allow multiple users to add documentation to - MediaWiki would suffice, I just wondered if anyone knew of any better alternatives (must be PHP).
I strongly suggest you run through the Wiki Choice Wizard over at WikiMatrix.org
It has user authentication built in to prevent unauthorized access.
I would say there are NO good alternatives to mediawiki other than Wordpress, another open source platform. Both have many extensions and are guaranteed to stick around for a very long time, probably forever. Here's an example of a simple intranet based on Wordpress (which I haven't used and don't recommend except as a guide to features) http://www.simpleintranet.org/?gclid=CJ_c7erQsbUCFa9aMgoduWsAug
Using anything that is not open source and heavily used for open content is asking for lots of downstream problems.
Do not be fooled by these "better" or newer or "more supported" wikis, they are all fly by night and will all be effectively gone in five years. Nothing protects you from some massive shift in underlying data structure even in something like Mindtouch which uses mediawiki as its base. You find lots of good information on how to customize mediawiki, e.g. http://lifehacker.com/5396832/customize-mediawiki-into-your-ultimate-collaborative-web-site
Other open source alternatives like tikiwiki had promise but simply aren't widely used and will either disappear or become front ends for the data standards set by mediawiki.
Many testimonials are available from major mediawiki users http://www.mediawiki.org/wiki/MediaWiki_testimonials
Now here's a particularly challenging application I doubt you'll be seeing on Confluence http://en.wikipedia.org/wiki/Intellipedia
I have used MediaWiki for about two years in the previous company, and had so much trouble with it. The notation is cumbersome, weak search engine, and pages have no clear hierarchy.
If you can afford a paid product, you should seriously consider looking into Confluence. If you require PHP, MySQL with a strong search engine consider having a look at LessonsLearnedServer. The application is easy to use, good quality and has a decent pricing scheme.
If you insist on free, my best experience was with WordPress. Although, you have to check what specific needs you require and check if they are provided by plugins.