I have an included file in php that can be include from inside a class method or from global scope. $this will be valid in the first case but not the the second.
Is there a way to check if $this is accessible? isset($this) and is_object($this) seem to return true even not being in a method of a class.
Well, when you include the file in a class, then $this is accessible. So you should do a more stringent type check here.
Try using instanceof to determine if $this is what you want:
if ($this instanceof MyObject) {
// in my class
} else {
// not in my class
}