Разница между $ a = & $ b, $ a = $ b и $ a = clone $ b в PHP OOP

Question

В чем разница между $a = &$b, $a = $b и $b = clone $a в PHP OOP? $a - это экземпляр класса.

Answer 1

// $a is a reference of $b, if $a changes, so does $b.  
$a = &$b; 

// assign $b to $a, the most basic assign. 
$a = $b; 

// This is for object clone. Assign a copy of object `$b` to `$a`. 
// Without clone, $a and $b has same object id, which means they are pointing to same object. 
$a = clone $b; 

и проверить больше информации с References, Object Cloning.

Answer 2

// $a has same object id as $b. if u set $b = NULL, $a would be still an object 
$a = $b; 

// $a is a link to $b. if u set $b = NULL, $a would also become NULL 
$a = &$b; 

// clone $b and store to $a. also __clone method of $b will be executed 
$a = clone $b; 

Answer 3

Если вы не знаете, что такое ZVAL структура, а что RefCount, is_ref в ZVAL структуре около, просто занять некоторое время для PHP's garbage collection.