Each PHP variable exists in a variable container called "Zval". A Zval variable container that includes two bytes of extra information in addition to the type and value of the variable. The first is "Is_ref", which is a bool value that identifies whether the variable belongs to a reference collection (reference set). With this byte, the PHP engine can differentiate between normal and reference variables, and since PHP allows users to use the custom reference by using &, there is an internal reference counting mechanism in the Zval variable container to optimize memory usage. The second extra byte is "RefCount", which represents the number of variables (also known as symbols) that point to the Zval variable container. All symbols exist in a symbol table, where each symbol has scope (scope), and those main scripts (for example, scripts that are requested by the browser) and each function or method also have scopes.
The object in PHP is passed as a reference
Is_ref = 0, refcount = 0 zval container will be destroyed at the end of script execution
Cite the official example
<?php$a = Array (' one '), $a  =& $a; Xdebug_debug_zval (' a ');
A: (refcount=2, Is_ref=1) =array ( 0 = (refcount=1, is_ref=0) = ' One ', 1 = (refcount=2, is_ref=1) = ...)
Executing unset$a will release the variables associated with Zval's memory but the closed loop itself still exists inside
(Refcount=1, Is_ref=1) =array ( 0 = (refcount=1, is_ref=0) = ' One ', 1 = (refcount=1, is_ref=1) = ...)
But at this point, no variable can be manipulated into the Zval container time has become a memory garbage does not release
Recycling mechanism: In a nutshell, after executing the script, the overall data of the remaining variables is all refcount-1, and if it is reduced to 0, it is determined that the garbage is destroyed by the memory container.