Garbage Collection

Question 1

What is a zval?

Answer 1

A container in which a PHP variable is stored. A zval container contains, besides the variable’s type and value, two additional bits of information. The first is called is_ref, and is a boolean value indicating whether or not the variable is part of a “reference set”. The second piece of additional information, called “refcount”, contains how many variable names (also called symbols) point to this one zval container.

Question 2

In a zval, if refcount is 1, then is_ref is…

Answer 2

…always FALSE.

Question 3

When do zvals get destroyed?

Answer 3

When their refcount reaches zero.

Question 4

When does a refcount get decremented?

Answer 4

When any symbol linked to the variable container leaves the scope (e.g. when the function ends), or when unset() is called on a symbol.

Question 5

How can you turn the garbage collecting mechanism on and off?

Answer 5

By calling gc_enable() and gc_disable() respectively.

Question 6

How does garbage collection affect performance?

Answer 6

Reduced memory usage and run-time slowdowns.