It is said that asp.net for example
hides the details.
Is that true? And could you say why?
Yes, it is true to a certain extent. In PHP you seldomly use proprietary markup like you do in ASP.NET or JSF for example. You most likely write (nearly) pure HTML and not some fancy XML in your view layer.
Then, in PHP, you are very close to the request that was issued, while frameworks like ASP.NET or JSF provide merely an abstraction of it. In PHP you can directly set headers and always access GET and POST variables easily, quite in contrast to both the other frameworks I mentioned, which are quite far away and thus it teaches you a tiny bit more about the http protocol.
Moreoever, PHP does not autogenerate view-tier code by default. Most high-level web frameworks do. If you only use high level abstraction frameworks, you might have problems when you are forced to deal with the details of web programming. I think PHP is a much better choice to learn about web programming than ASP.NET is.