<?php $a = 1; $b =& $a; unset($a); ?>
won't unset $b, just $a.
Again, it might be useful to think about this as analogous to the Unix unlink call.
When PHP parses a file, it looks for opening and closing tags, which are <?php and ?> which tell PHP to start and stop interpreting the code between them.
Variables in PHP are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive. Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.