php unset（）它到底是做什么的？

Question

The PHP documentation states the following about the unset() language construct:

destroys the specified variables.

Unset a given variable

What does this really mean? Just destroy the variable content; empty out a variable and give it the value null? Or does it mean, wipe out the variable not just it's content but everything?

I wrote the following:

$a = "tom";

unset($a);

var_dump($a);

and got this:

Notice: Undefined variable: a in...

Answer 1

It destroys the content, and leaves the variable in an undefined state.

If you try to access the variable again after it's unset, and you have warnings disabled, you might be able to retrieve the value, but it's absolutely not guaranteed.