it's my way to remember.
<?php
// the var $a is point to the value 1, as a line connect to value 1
$a = 1;
// the var $b point to the value which the var $a point to, as a new line connect to value 1
$b =& $a;
// cut the line of the var $a to value 1,now $a is freedom,it's nothing point to. so the value of $a is null
unset($a);
?>
$a--------> 1
↑
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|
$b