You can stick with PHP if you want. There are QT bindings, GTK bindings, OSX/Cocoa bindings, and you can call Win32 functions. I don't know how stable all of those are, but you can do GUI in PHP as well as the command line stuff.
As for other languages... PHP is very C like. It started as basically a scripting wrapper around C (IIRC), which is why you have functions named after the C standard library (like strstr). C like languages will feel quite familiar.
I would think Python would probably be the closest to PHP. It's a scripting language, syntax is somewhat close, it has a ton of libraries, and is very well supported and commonly used. I'd imagine it would feel pretty familiar. Using indenting instead of brackets for blocks can throw some people, but it fit the way I already indented my code.
Ruby is quite popular, and is also a scripting language. I think it's farther away, syntax wise, than Python, but I've never really used it so I can't promise that. I know it has at least GTK bindings.
Perl has a lot of resources and bindings, but isn't as easy to read as PHP since you have to learn the special variables like @_. It was never really my cup of tea.
You do have the C/C++ stuff, and Java has it's large library. You may want to go that way since you say you've used C++ before. If you're on a Mac (or willing to use GNUStep) you could go with Objective-C/Cocoa. That's getting rather far from PHP syntax though.
All of these languages have database connections. You don't mention what platform you're on.
But for easy to work with, quick to pick up, works all sorts of places, and can definitely do GUIs... Python would be a good choice to look at.