[ot]My biggest gripe with PHP and multi-threaded applications is they aren't out of the box. I find it more often than not, I need it (probably because I work on such odd projects that either do a LOT of data crunching or have the ability to make good use of parallel processing).
In .NET it is SO simple to spawn multiple processes and then wait for all to finish before proceeding (once you get to the point where the results are needed). With PHP, you have a LOT of hoops to jump through before you can even begin to start that. You have to install and test multiple components that try and implement it, you then have to configure it for each environment, and so on. It just irritates me to no end.
I'm also not a huge fan of mixing and matching languages in a project unless there is a significant boost in performance or flexibility to meet the requirements in a short time frame. Having a system that is fluid in one language makes it easier on devs to be able to quickly work in all parts, versus, if your async processing is in one language and your presentation another, you could be siloing your devs (again personal opinion).
I really don't think it would be THAT difficult for PHP to properly implement asynchronous operations, especially since they are pushing OOP. Take a look at the Parallel Library in .NET, that made things DEAD simple and it is very powerful.[/ot]
In the end, (no offense to you @neotericuk ; ), the system you run on should be determined before your project really is off the ground. You should be developing against it, testing against it, and eventually deploying to it. You should be using the requirements to determine what language suits it best, taking into consideration the tools that are available to each system, etc. Certain languages are good at specific tasks, others are kind of a hodge podge and can be utilized for just about any project (though they may suffer a few performance hits here and there if they weren't specifically designed for what you were attempting; or require a lot of additional components...).