PHP的内存问题（5）

Question

Does calling unset() free the memory that was associated with that object? There are a couple cases where I find myself handling large associative arrays and I would like to remove them when done (freeing up memory to create new ones).

Answer 1

Does calling unset() free the memory that was associated with that object?

Yes it does.

Check for yourself using memory_get_usage():

echo memory_get_usage() . "<br />";
unset($array['key']);
unset($array['key2']);
unset($array['key3']);
echo memory_get_usage();

More Resources:

• Garbage Collection (Offical Docs but for 5.3)
• Be wary of garbage collection

Quoting from later link:

PHP performs garbage collection at three primary junctures:

1. When you tell it to
2. When you leave a function
3. When the script ends

Situation 1 occurs when you use unset(), mysql_free_result(), or other resource-destroying functions that explicitly clear up after your variables. Situation 2 clears up resources implicitly - any variable that leaves scope, i.e. is no longer applicable, gets cleared up for you. Finally, situation 3 frees up all script-related resources implicitly.