The reason of this bug is IS_CV variables.
PHP doesn't increment/decrement refcount during fetching, also the order of fetches may be changed. In 5.1 and lvalue of "$a[] = $a" is evaluated before rvalue and changes rvalue (addes new element to array). Then $a is assigned into this element. As a result we got circular data structure where we shouldn't.

The same bug occurs with list()

Reproduce code:
---------------
<?php
$b = array("1","2","3");
list($a, $b, $c) = $b;
var_dump($a); //should print "1" (not "2")
var_dump($b);
var_dump($c);
$b = array("1","2","3");
list($a, $b[0], $c) = $b;
var_dump($a); //should print "1" (not "2")
var_dump($b);
var_dump($c);
?>

Expected result:
----------------
string(1) "1"
string(1) "2"
string(1) "3"
string(1) "1"
array(3) {
  [0]=>
  string(1) "2"
  [1]=>
  string(1) "2"
  [2]=>
  string(1) "3"
}
string(1) "3"

Actual result:
--------------
string(1) "2"
string(1) "2"
string(1) "3"
string(1) "2"
array(3) {
  [0]=>
  string(1) "2"
  [1]=>
  string(1) "2"
  [2]=>
  string(1) "3"
}
string(1) "3"

Seems this bug affects comparision of objects which contains references to themself:

$cat >comparision.php
<?php

class A{
	private $a;
	
	public function __construct()
	{
		$this->a = $this;
	}
}

$a = array(new A, 1);
$b = array(new A, 1);

var_dump($a == $b);

?>
^C

$php comparision.php 

Fatal error: Nesting level too deep - recursive dependency? in /users/ssmirnova/comparision.php on line 15

Nowadays we got the expected result.

array(1) {
  [0]=>
  array(0) {
  }
}