php ::Array references

any idea why

foreach ($groups as &$group)
  $group = trim(str_replace(',', '', $group));

echo '<pre>';
print_r($groups);
echo '</pre>';

$groupsq = $groups;
foreach ($groupsq as &$group)
  $group = '\'' . $group . '\'';

echo '<pre>';
print_r($groups);
echo '</pre>';

Yields

Array
(
    [0] => Fake group
    [1] => another group
    [2] => non-existent
)
Array
(
    [0] => Fake group
    [1] => another group
    [2] => 'non-existent'
)

The part i am interested in is,

Why does the second array modification effect the last item on the first array?