memory - How important is it to unset variables in PHP?

Question

I am somewhat new to PHP, and I am wondering:

How important is it to unset variables in PHP?

I know in languages like C, we free the allocated memory to prevent leaks, etc. By using unset on variables when I am done with them, will this significantly increase performance of my applications?

Also, is there a benchmark anywhere that compares the difference between using unset and not using unset?

Answer 1

See this example (and the article I linked below the question):

$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>
";      // 120172
echo memory_get_peak_usage() . "<br>
"; // 121248

$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>
";      // 120172
echo memory_get_peak_usage() . "<br>
"; // 201284

As you can see, at one point PHP had used up almost double the memory. This is because before assigning the 'x'-string to $x, PHP builds the new string in memory, while holding the previous variable in memory, too. This could have been prevented with unsetting $x.

Another example:

for ($i=0; $i<3; $i++) {
    $str = str_repeat("Hello", 10000);
    echo memory_get_peak_usage(), PHP_EOL;
}

This will output something like

375696
425824
425824

At the first iteration $str is still empty before assignment. On the second iteration $str will hold the generated string though. When str_repeat is then called for the second time, it will not immediately overwrite $str, but first create the string that is to be assigned in memory. So you end up with $str and the value it should be assigned. Double memory. If you unset $str, this will not happen:

for($i=0;$i<3;$i++) {
    $str = str_repeat("Hello", 10000);
    echo memory_get_peak_usage(), PHP_EOL;
    unset($str);
}

// outputs something like
375904
376016
376016

Does it matter? Well, the linked article sums it quite good with

This isn't critical, except when it is.

It doesn't hurt to unset your variables when you no longer need them. Maybe you are on a shared host and want to do some iterating over large datasets. If unsetting would prevent PHP from ending with Allowed memory size of XXXX bytes exhausted, then it's worth the tiny effort.

What should also be taken into account is, that even if the request lifetime is just a second, doubling the memory usage effectively halves the maximum amount of simultaneous requests that can be served. If you are nowhere close to the server's limit anyway, then who cares, but if you are, then a simple unset could save you the money for more RAM or an additional server.