References Explained
Predefined variables
References Explained
PHP Manual

## Spotting References

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:

### global References

When you declare variable as global $var you are in fact creating reference to a global variable. That means, this is the same as:

`<?php
$var =& $GLOBALS["var"];
?>`

That means, for example, that unsetting $var won't unset global variable.

### $this

In an object method, $this is always a reference to the caller object.

References Explained
Predefined variables
References Explained
PHP Manual