PHP array swap example.PHP saves memory when passing arrays by value and creating new ones in different order.Only symbols' table is updated.

array-swap.php

<?php
$a = [1,2];
function f($b) {
    xdebug_debug_zval('b');
    return [$b[1], $b[0]];
}
xdebug_debug_zval('a');
$c = f($a);
xdebug_debug_zval('c');

// a: (refcount=1, is_ref=0)=array (0 => (refcount=1, is_ref=0)=1, 1 => (refcount=1, is_ref=0)=2)
// b: (refcount=3, is_ref=0)=array (0 => (refcount=1, is_ref=0)=1, 1 => (refcount=1, is_ref=0)=2)
//              ^ as call by reference

// c: (refcount=1, is_ref=0)=array (0 => (refcount=2, is_ref=0)=2, 1 => (refcount=2, is_ref=0)=1)
//                                                 ^   swapped, but saving memory ^                                                   

class-clone.php

<?php
class A { public $b; public $c; public $d; }

$a1 = new A();
$a1->b = "SOME BIG DATA";
xdebug_debug_zval('a1');
//a1: (refcount=1, is_ref=0)=class A { public $b = (refcount=1, is_ref=0)='SOME BIG DATA'; public $c = (refcount=1003, is_ref=0)=NULL; public $d = (refcount=2, is_ref=0)=NULL }
//              ^ oobject link                               ^ First copy                                        ^ every null pointer

$a2 = clone $a1;
xdebug_debug_zval('a1');
// a1: (refcount=1, is_ref=0)=class A { public $b = (refcount=2, is_ref=0)='SOME BIG DATA'; public $c = (refcount=1004, is_ref=0)=NULL; public $d = (refcount=3, is_ref=0)=NULL }
//               ^ unique object link                         ^ Not copying big data

$a2->c = "Changed variable";
$a3 = clone $a2;
xdebug_debug_zval('a3');
// a3: (refcount=1, is_ref=0)=class A { public $b = (refcount=3, is_ref=0)='SOME BIG DATA'; public $c = (refcount=2, is_ref=0)='Changed variable'; public $d = (refcount=4, is_ref=0)=NULL }
//                                                            ^ a1, a2 and a3 shares this data                    ^ a2 and a3 shares