Suppose I have a fairly generic question (like about the use of global variables or something like that) but I'm looking for an answer related to PHP, should I include that requirement in the title? I know that it's what tags are for, but maybe it helps visibility and relevance in external engine (google) searches?
I only do this when I'm asking the type of question that's liable to get answered by people who haven't actually read the question or the tags.
For example, I asked "Is SQL Server DRI slow?" and actually got a very good answer, but I suspect that if I hadn't added the "SQL Server" part then a bunch of people might have given irrelevant answers for other DBMSes (and the specific DBMS was the issue there). If I had just asked "Is DRI slow?" then the title would have suggested a product-agnostic question; aside from including the product name, there was no way to reword it to be more specific.
On the other hand, I also asked about "Approaches for generic, compile-time safe lazy-load methods", and even though the verbiage does not suggest a specific language, I think it's obvious to any reader that the language matters.
Honestly, it can't hurt to include the product name, but as dmckee says, just don't make it a "pseudo-tag" at the beginning of the question; make your title a coherent sentence that includes the product name in its proper place (i.e. "Is there an XYZ library available for .NET?").
Keep tags in the tags, please.
You don't need the additional visibility. People interested in PHP likely already have that in their interesting tags.
People interested in not seeing PHP questions likely already have that in their ignored tags list.
Neither group will be served by you placing the tag in the title.
If you start to get answers from people who didn't notice the tag, then maybe you should consider editing the title to include the tag (without the brackets).
Still, I hope you won't need to change the title from "How do I Consume a WCF Service using nuSOAP" to "How do I Consume a WCF Service using nuSOAP in PHP".
If PHP is a requirement, you can certainly mention it in the title, but nobody can force you to do so. You can use any title you wish. If people think that it's not sufficiently precise, it will get edited anyway. That being said, some people do overlook tags, so the more precise your question title (and body) is, the better.