Problems caused by unset destroy variables in PHP

First, let's look at an example:

The code is as follows	Copy Code
<?php $a = "Hello Springload"; $b = $a; Unset ($b); echo $a; Hello Springload ?>

This is clear to us, unset ($b), just disconnected the variable name and worth binding, but an interesting question arises:

The code is as follows	Copy Code
<?php $a = "Hello Springload"; $b = $a; unset ($a); echo $b;//hello springload ?>

Why $a destroyed, why is the value of $b still in it?

Note: the unset () function is only released if all variables pointing to the value (for example, if the reference variable points to that value) are destroyed, as follows:

code is as follows	copy code
<?php     $a = "Hello Springload";     $b = $a;     unset ($a);     unset ($b);     echo $b;/output null