PHP release memory of include file - php

Question

Can I release the memory generated from the included file, here are my codes
a.php
<?
echo memory_get_usage();
include_once "b.php";
echo memory_get_usage();
$n = new obj();
echo memory_get_usage();
?>
b.php
<?
class obj {
protected $_obj = array{
....
}
function ....
}
?>
I checked that, after I include the b.php, the memory use will increase, which is more higher than create a new object. The result is as below
348832
496824
497072
So, how can I release the included file's memory?

Answer 1

I think PHP cannot de-include (I mean, free the memory space hold by included file) since the contents of the file may be used later. This is a design choice of PHP creators.
After your PHP script finishes, it will free consumed memory, do not worry about it too much unless it really makes too overhead and you have a high volume traffic load.
If there is (let's say a huge) object coming from included file you want to deallocate right now, use unset($obj). It will help some. You should read more about PHP Garbage Collection policy to have a fine tuning.

Answer 2

PHP compiles the code from all your included/required files to opcode for faster execution, this memory cannot be de-allocated, php frees it when script finishes.
If you allocate some memory / object within your second required file, it will take the memory too, but you can unset those variables (but this is not your case, since you are just declaring a class within your b.php).
Also, php must know, that you don't want to include file b.php again (include_ONCE), so it keeps internal record of files which you have included to not try to include them again (that means this also consumes memory).
As ahmet alp balkan said, you can also try to keep memory usage of your script the lowest possible if you deallocate variables that you don't need anymore via unset();
But for performance reasons, PHP doesn't de-allocate this memory right in the moment when you call unset, but rather marks this unsetted variable as "freed".
Then garbage collector comes and frees all freed variables (+ those ones it thinks you won't need anymore). GC is triggered over time.
Try for example this:
<?
echo memory_get_usage();
include_once "b.php";
echo memory_get_usage();
$n = new obj();
echo memory_get_usage();
unset($n);
echo memory_get_usage();
// try to wait for GC
sleep(5);
echo memory_get_usage();
?>

Answer 3

If there is a real danger of running out of memory and you only need to extract specific information from the file, you can use $x = file_get_contents() inside a function or method, then extract the information with preg_match().
This will cost you speed, but $x should be released when the function or method returns. It has the further advantage that the memory taken by the file will not be used at all if the function or method is never called. For example:
/* You need the value of $modx->lang_attribute and there is something like this
in the file: $modx_lang_attribute = 'en'; */
$x = file_get_contents('path/to/file');
$pattern = "/modx_lang_attribute\s*=\s*'(\w\w)'/";
preg_match($pattern, $x, $matches);
return isset($matches[1])? $matches[1] : 'en';
In some cases, you can save more memory by processing the file line by line.
The down side of this is that the file will not be tokenized, so it will take up more memory while in use, but at least you won't be carrying it around for the rest of the program.