The reason we ended up learning php at my company was pure market demand. We kept getting prospects whose websites written in php and we either learned and got the business or we said no and missed out. There is a ton of php out there, and if you want to build your business up on consulting revenue, its hard not to use it fairly regularly.
I think this is very subjective to where in the world you are. In the UK, especially in larger businesses, PHP is rare and C# / ASP.NET / .NET are fairly very common. Rates for PHP are also generally lower.
As an example Jobserve.com, one of the major search engines for jobs here, has 501 PHP jobs, 1,100 ASP.NET jobs, 2,736 .NET jobs and 1,954 C# jobs (search against UK, Europe and Middle East but most of those will be UK).
Whilst I like you logic I don't think this is the case, at least in the UK market. Trying to find a PHP developer is actually very difficult here, especially if the work is at a somewhat boring company, I guess they want to be at start-ups. No lack of unemployed .NET devs (or employed and looking for a change), although a real lack of quality ones, but I would imagine that is the case with most dev jobs.
