Exploring PHP Variables, recount, and copy on write.

variable-refcount.php

<?php

add_action( 'init', function() {

	/**
	 * 1. PHP adds the variable into the symbols table.
	 * 2. PHP adds the value into the data table, which is called zval.
	 * 3. PHP points the value location to the variable location, binding them together.
	 * 4. refcount is incremented to 1.
	 */
	$item_1 = 'something';

	/**
	 * Let's look at the zval for the variable $item_1:
	 *
	 * The output is:
	 *
	 * item_1:
	 * (refcount=1, is_ref=0),string 'something' (length=9)
	 *
	 * Notice the value is 'something' and the refcount is 1, meaning 1 variable is pointing to that memory location.
	 */
	xdebug_debug_zval( 'item_1' );

	/**
	 * 1. PHP adds the variable into the symbols table.
	 * 2. PHP points the variable to the existing zval, i.e. value in memory.
	 * Why?  To optimize memory usage.  Instead of creating a new memory location (zval) for the value, it points
	 * to the same value that $item_1 is pointing to.
	 * 3. refcount for the value is incremented to 2.
	 */
	$item_2 = $item_1;

	/**
	 * Let's look at the zval for the variable $item_1.  The output is:
	 *
	 * item_1:
	 * (refcount=2, is_ref=0),string 'something' (length=9)
	 *
	 * Notice the value is 'something' and the refcount is 2, meaning 2 variables are pointing to that memory location,
	 * i.e. $item_1 and $item_2.
	 */
	xdebug_debug_zval( 'item_1' );
	/**
	 * Let's look at the zval for the variable $item_2.  The output is:
	 *
	 * item_2:
	 * (refcount=2, is_ref=0),string 'something' (length=9)
	 *
	 * Notice that's the same as $item_1. Why? They are both pointing to the exact same value memory location.
	 */
	xdebug_debug_zval( 'item_2' );

	/**
	 * Now let's change the value in one of the variables and see what happens.
	 *
	 * 1. PHP creates a new memory location in the data table, i.e. creates a new zal container.
	 * 2. It then points $item_1's memory location in the symbols table to the new location of where the value
	 * lives in memory (the new zval).
	 * 3. It increments the new value's refcount to 1, as only one variable is pointing to it.
	 * 4. It decrements the old value's refcount by 1, as $item_1 is no longer pointing to it.
	 *
	 * This process is known as Copy on Write.  It's used to optimize memory.
	 */
	$item_1 = 'something else';

	/**
	 * Let's look at the zval for the variable $item_1.  The output is:
	 *
	 * item_1:
	 * (refcount=1, is_ref=0),string 'something else' (length=14)
	 *
	 * Notice the recount and value match what we'd expect.
	 */
	xdebug_debug_zval( 'item_1' );
	/**
	 * Let's look again at the zval for the variable $item_2.  The output is:
	 *
	 * item_2:
	 * (refcount=1, is_ref=0),string 'something' (length=9)
	 *
	 * Notice that the refcount is 1 and the value didn't change.
	 */
	xdebug_debug_zval( 'item_2' );

	die();

});