php ::Array references

Question

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?

Answer 1

First, you need to clean up the references after each foreach loop using unset(), like so:

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

unset($group);

// ...

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

unset($group);

Secondly, you're printing $groups instead of $groupsq:

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

The last item of $groups is being modified because you didn't clean up the reference after the first foreach loop.

Answer 2

Here is an in-depth article explaining the technical details behind this behavior: http://schlueters.de/blog/archives/141-References-and-foreach.html