I'm hoping to use this project to really push my PHP learning
is not a good reason for your employers to choose PHP.
Without knowing the details of the application you are trying to build, the infrastructure on which it will run, the skillsets already supported within your organisation, etc etc no-one will be able to give you any answer to this.
Could you do your own research? If I was paying your salary, that's what I'd expect.
It's all very well for someone here to identify cases where PHP code is more efficient than ASP, or where particular libraries make a certain task easier, or whatever other reason they might chose to use PHP, but that's only going to be of partial relevance to your employer.
If your employer already as a load of code written in ASP, and has a team of developers, operations staff and support staff who are well-versed in that technology, using a different technology for this application is a very major risk for them.
The same would apply if everything else was built on a J2EE framework. Why introduce different technologies, just forthe sake of it?
For large companies who can afford to have all the backup techs they need, then developing on multiple platforms can make sense as the risks of exposure to vulnerabilities is somewhat diluted. hackers exploiting known weakneses in a particular technology won't be able to break everything.
For small companies with just a handful of tech folks, then it makes sense to play to their strengths, and focus on one technology.
Note, I say all of the above as someone who thoroughly respects PHP as a language and does quite a bit of development in it.
