Is it good idea to switch from Asp.net to PHP?

All right, here is very brief consideration. Everything depends on your purpose.

PHP is a very cheap thing. It is not anything serious or well designed. To me, it just looks like an ersatz language, not something real. Perhaps this is one of the reasons of its popularity. And yet, I've uses it a bit. Why? Because it's everywhere and is provided even with the cheapest (or free of charge) and dirtiest Web hosting packages. In other words, if you have nothing on the server side, you can always use PHP.

ASP.NET is about the opposite side of the scale; it's probably the most robust technology. But it's harder to get, hosting tends to cost more, and so on. Also, nearly all hosting options are about IIS, which is not that light weight as Apache, uses more resources; and I think Apache performance is better. (There is ASP.NET for Apache, mod_mono, but it is rarely used.) However, 1) by my subjective estimate, I would say the quality per dollar value is much better; 2) with time, the situation shifts quickly in favor of ASP.NET.

If your question, there is a troublesome note, and this will be the most important part of my answer. "Switch from ASP.NET to PHP". "Salary"… Switch, really? What does it mean? Do you think that you are choosing a career? Or do you think working with PHP would force you to forget .NET forever? (Well, some brain damage may occur if you take poor technology too seriously, so be careful :-).) For a real software developer, another technology or language is not career, it's just spending couple of days and learning one more little thing, and then working with it a bit, when and if circumstances gives you some benefits from such work. Big deal! The worst thing would be a waste of your time.

—SA