Last updated on February 18th, 2022 at 06:29 pm
Fix PHP memory usage issues , Solution for out of memory
Unset variables in PHP and memory usage related issues can be resolved up to a greater extend. This section gives you a fairly good idea on the advantage of upsetting variables in php and how it helps in freeing up memory.
You might also be interested in, How to find memory usage of a PHP Script.
$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>\n"; // 120172
echo memory_get_peak_usage() . "<br>\n"; // 121248
$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>\n"; // 120172
echo memory_get_peak_usage() . "<br>\n"; // 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 $strand 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);
}
Output
375904
376016
376016
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 withAllowed 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.
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