I think the question whether PHP+DB or DotNetNuke becomes whether it will be better to build an application "by hand" using a general purpose language, or to use a CMS solution. If the latter, is DotNetNuke the best CMS for the job?

PHP is a general-purpose language and DotNetNuke is a CMS built with general purpose VB.NET (Visual Basic) so the original comparison is like "apples and bacon strips".

DotNetNuke seems to have generated a fair amount of enthusiasm (~125,000 programmers) but also is fairly new in the CMS game, and has scads of competitors, so it's popularity could fade. I don't see much in the way of support for special-purpose database programming, but you can always do that in VB.NET. One plus for DotNetNuke is that you can purchase support.

I have some experience programming with pre-.NET VB, and some experience with PHP. I would say that PHP is a bit easier to learn and is more popular, hence easier to find humans with expertise. But I could be wrong.

In one sense DotNetNuke is Open Source, but the platform that it runs on, .NET Framework , is not free. I assume you also would have to pay for VB.NET, unless your subcontractor owns a copy. An additional limitation is that it will only run on a Microsoft Windows server. Here is what the consortium's web site says about DotNetNuke:"DotNetNuke is an open-source content management system ideal for creating and deploying projects such as commercial websites, corporate intranets and extranets, and online publishing portals."

There is a nifty tool for comparison of CMS solutions. You can compare DotNetNuke and PHP Nuke for example. And here's an interesting thread comparing Rainbow and DotNetNuke that mentions PHP- Re: Rainbow or DotNetNuke?

The choice of tools will of course depend on your needs.