Daily PHP WTF. Adding a key to an array creates a new and different array.

array-reference.php

<?php

$a = array();
$b = $a;

// "When using the identity operator (===), object variables are identical 
// if and only if they refer to the same instance of the same class."
if ($a === $b) print "same\n";

$a['test'] = 'something';

if ($a === $b) 
  print "same\n";
else
  print "No longer the same object reference\n"; // Guess what, $a doesn't point to $b any more


// Let's try that again
$a = array();
$b = &$a; //  use ampersand to explicitly make reference a pointer

if ($a === $b) print "same\n";

$a['test'] = 'something';

if ($a === $b) 
  print "same\n"; // now they point to the same thing!
else
  print "No longer the same object reference\n";