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Assigning object instances results in assigning references:

<?php
class s {
  public $state = 1;
}

$s = new s();
$t=$s;

$s->state=4;

echo "s state ".$s->state."\n";
echo "t state ".$t->state."\n";

s state 4
t state 4

Using clone creates a shallow copy of the object. (Add a __clone() method to define your own deep copy. See http://php.net/manual/en/language.oop5.cloning.php)

<?php

class s {
  public $state = 1;
}

$s = new s();
$t=clone $s;

$s->state=4;

echo "s state ".$s->state."\n";
echo "t state ".$t->state."\n";

s state 4
t state 1

Note: Use debug_zval_dump() to see the reference count for a variable.