Edit:
see my example below. this post was confusing.
no, there isn't really such a keyword. basically, everything is in the global scope unless it's in some other scope :D
$this only works from within a class (object).
so, whatever scope you're in when you assign a variable is the scope it's available in. AFAIK, you can't assign a variable to a particular scope while you're inside another. In the global scope (outside all functions, classes, namespaces), $myVar = 'bob'; creates a global variable. Inside a function/class/namespace, it creates a local variable. If you're inside a function and you want to reference a variable in the global scope, you can use $GLOBALS['myVar'] (references $myVar) or declare global $myVar; (from inside the local scope; makes $myVar available in the current, local scope).
kinda counter-intuitive, IMHO... I would have made it so you could create a variable in the global scope, declare it global, and then have it automatically available in all scopes... but oh well. it is what it is. that's basically what $GLOBALS does, I guess.