You have already seen basic variable declarations:
Note that neither value needs to be previously declared, and that PHP determines the variable type at runtime by context. Variable names are case sensitive, so "$foobar" isn't the same as "$FooBar". Variables can be integers, floating point numbers, arrays (scalar, associative, or multi-dimensional), strings, or objects. The two examples above are an integer and a string, and that's all I'll be using in this short introduction.