What does PHP assignment operator do?

I happens 开发者_如何学Pythonto read this http://code.google.com/speed/articles/optimizing-php.html

It claims that this code

$description = strip_tags($_POST['description']);
echo $description;

should be optimized as below

echo strip_tags($_POST['description']);

However, in my understanding, assignment operation in PHP is not necessarily create a copy in memory.

This only have one copy of "abc" in memory.

$a = $b = "abc";

It consumes more memory only when one variable is changed.

$a = $b = "abc";
$a = "xyz";

Is that correct?

should be optimized as below

It's only a good idea if you don't need to store it, thereby avoiding unnecessary memory consumption. However, if you need to output the same thing again later, it's better to store it in a variable to avoid a another function call.

Is that correct?

Yes. It's called copy-on-write.

In the first example, if the variable is only used once then there is not point of making a variable in the first place, just echo the statements result right away, there is no need for the variable.

In the second example, PHP has something called copy on write. That means that if you have two variables that point to the same thing, they are both just pointing at the same bit of memory. That is until one of the variables is written to, then a copy is made, and the change is made to that copy.

The author does have a point insofar as copying data into a variable will keep that data in memory until the variable is unset. If you do not need the data again later, it's indeed wasted memory.

Otherwise there's no difference at all in peak memory consumption between the two methods, so his reasoning ("copying") is wrong.