Calling unset() in PHP script - php

Question

Coming from a C/C++ background, I am used to doing my own garbage collection - i.e. freeing resources after using them (i.e. RAII in C++ land).
I find myself unsetting (mostly ORM) variables after using them. Are there any benefits of this habit?
I remeber reading somewhere a while back, that unsetting variables marks them for deletion for the attention of PHP's GC - which can help resource usage on the server side - true or false?
[Edit]
I forgot to mention, I am using PHP 5.3, and also most of the unset() calls I make are in a loop where I am processing several 'heavy' ORM variables

Answer 1

I find that if your having to unset use alot your probably doing it wrong. Let scoping doing the "unsetting" for you. Consider the two examples:
1:
$var1 = f( ... );
....
unset( $var1 );
$var2 = g( ... );
....
unset( $var2 );
2:
function scope1()
{
$var1 = f( ... );
....
} //end of function triggers release of $var1
function scope2()
{
$var2 = g( ... );
....
} //end of function triggers release of $var2
scope1();
scope2();
The second example I would be preferable because it clearly defines the scope and decreases the risk of leaking variables to global scope (which are only released at the end of the script).
EDIT:
Another things to keep in mind is the unset in PHP costs more (CPU) than normal scope garbage collection. While the difference is small, it goes to show how little of an emphases the PHP team puts on unset. If anything unset should give PHP insight that on how to release memory, but it actually adds to execution time. unset is really only a hack to free up variables that are no longer needed, unless your doing something fairly complex, reusing variables (which acts like a natural unset on the old variable) and scoping should be all you ever need.
function noop( $value ){}
function test1()
{
$value = "something";
noop( $value ); //make sure $value isn't optimized out
}
function test2()
{
$value = "something";
noop( $value ); //make sure $value isn't optimized out
unset( $value );
}
$start1 = microtime(true);
for( $i = 0; $i < 1000000; $i++ ) test1();
$end1 = microtime(true);
$start2 = microtime(true);
for( $i = 0; $i < 1000000; $i++ ) test2();
$end2 = microtime(true);
echo "test1 ".($end1 - $start1)."\n"; //test1 0.404934883118
echo "test2 ".($end2 - $start2)."\n"; //test2 0.434437990189

Answer 2

If a very large object is used early in a long script, and there is no opportunity for the object to go out of scope, then unset() might help with memory usage. In most cases, objects go out of scope and they're marked for GC automatically.

Answer 3

Yes it can especially when you are dealing with big arrays, and script require much time to run.

Answer 4

Without going to look up some proof I'm going to say that it doesn't really matter. Garbage collection occurs automatically when you leave a function or a script ends. So unless you are really strapped for resources, don't bother.
OK, looked something up. Here is a good quote:
"Freeing memory - particularly large
amounts - isn't free in terms of
processor time, which means that if
you want your script to execute as
fast as possible at the expense of
RAM, you should avoid garbage
collection on large variables while
it's running, then let PHP do it en
masse at the end of the script."
For more info on the subject check out the links provided in the first answer here.

Answer 5

I thought PHP variables were only preserved through the lifetime of your script, so it's unlikely to help that much unless your script is particularly long-running or using a lot of temporary memory in one step.
Clearing explicitly may be slower than letting them all be automatically cleared at startup.
You're adding more code, which is generally going to make thing slower unless you know that it helps.
Premature optimization, in any case.

Answer 6

The PHP GC is usually good enough so that you usually do not need to call unset() on simple variables. For objects however, the GC will only destroy them when they leave scope and no other objects refer to them. Unset can help with memory in this case. See http://us3.php.net/manual/en/language.references.unset.php

Answer 7

I have had to use unset when you are running into memory issues when looping through and making copies of arrays. I would say don't use it unless you are in this situation ad the GC will kick in automatically.