Many syntax constructs in PHP are implemented via referencing
mechanisms, so everything told above about reference binding also
apply to these constructs. Some constructs, like passing and
returning by-reference, are mentioned above. Other constructs that
use references are:
When you declare variable as global $var you
are in fact creating reference to a global variable. That means,
this is the same as:
That means, for example, that unsetting $var
won't unset global variable.
In an object method, $this is always reference
to the caller object.
Unsetting References