It's called when the first of these conditions are met:
- The reference count of the object goes to 0 (these usually happens when the object has no more variables that reference it -- they were unset or went out of scope --, but it can happen later, as an object may be referenced by something other than a variable -- in fact, the reference count is just a number and can be manipulated in an arbitrary way).
- When using PHP 5.3, when the garbage collector detects the positive reference count is due to circular references.
- Otherwise, when the script finishes cleanly.
In short, you should not rely on it always being called, because the script may not finish cleanly.