fredemmott-pro:~ fredemmott$ cat test.php
<?php

$a = [123, 456];
foreach ($a as &$item);
$b = array_combine($a, $a);
var_dump($a);
var_dump($b);
fredemmott-pro:~ fredemmott$ php test.php
array(2) {
  [0]=>
  int(123)
  [1]=>
  &int(456)
}
array(2) {
  [123]=>
  int(123)
  [456]=>
  &int(456)
}
fredemmott-pro:~ fredemmott$ hhvm test.php

Warning: Invalid operand type was used: Invalid type used as key in /Users/fredemmott/test.php on line 5

Warning: Invalid operand type was used: Invalid type used as key in /Users/fredemmott/test.php on line 5
array(2) {
  [0]=>
  int(123)
  [1]=>
  &int(456)
}
array(0) {
}