Well, it's personal preference really, just like camelCase is.
But according to the people behind C++, general programing standards suggest that you should do so.
Sorry, I am a C++ programmer but I have heard no such thing. The C++ Standard does not follow such a naming convention. Speaking of "general programming standards", Sutter and Alexandrescu's C++ Coding Standards recommend: "Don't overlegislate naming, but do use a consistent naming convention". They do suggest a simple naming convention to use if you cannot decide on your own naming convention.
...and I know that C++ is not php, but since we have adopted camelCase, it seems logical to adopt this style for static methods (or functions) in php doesn't it?...
No. As Weedpacket pointed out, "they're always prefixed by a class identifier". Since the style makes no difference for readability, this could even be a case of overlegislating naming. If you do want to enforce something, enforce prefixing the class name, since PHP does allow static methods to be called using the syntax of non-static methods.
...Especially since, now we're using webservices in php, we should adopt the general programing standard for ease of compatibility etc, wouldn't you say?
There is no such thing as "the general programming standard" where style is concerned. There are only style-related programming standards specific to an organisation, be it a team, a project, a company, or even a standards committee. Again, Sutter and Alexandrescu state: "Say only what needs saying: Don't enforce personal tastes or obsolete practices."
Furthermore, they also advise:
Do use consistent formatting within each source file or even each project, because it's jarring to jump around among several styles in the same piece of code. But don't try to enforce consistent formatting across multiple projects or across a company.
In our context, this translates to: recommend that users be consistent with a reasonable formatting style when posting, but do not enforce a particular formatting style.