Why does a PHP script constantly take up more memory?

Question

See this example:

echo memory_get_usage() . "\n"; // 36640
$a = str_repeat("Hello", 4242);
echo memory_get_usage() . "\n"; // 57960
unset($a);
echo memory_get_usage() . "\n"; // 36744

Can anyone explain why after un-setting the variable the memory usage does not return to 36640

Answer 1

If you do it twice the memory will stay at 36744...

echo memory_get_usage() . "\n"; // 36640
$a = str_repeat("Hello", 4242);
echo memory_get_usage() . "\n"; // 57960
unset($a);
echo memory_get_usage() . "\n"; // 36744
$a = str_repeat("Hello", 4242);
unset($a);
echo memory_get_usage() . "\n"; // -> 36744

Answer 2

Garbage collection is an expensive operation, even if there's only a single variable to unset. PHP won't run the collector each time you unset a var, as that'd waste a huge amount of CPU time.

PHP will only run the collector when it has to, as in when something wants more memory than is available.