Description:
------------
This regression was introduced in 7.0.x, and worked perfectly otherwise in previous versions (5.6.x).

It can be reproduced with "default:" case only.

Test script:
---------------
<?php
function check_data(&$var) {
        if (is_array($var)) {
                foreach ($var as $key => $val) {
                        check_data($var[$key]);
                }
        }
}

$data = ['values' => [['type' => '4']]];
check_data($data);
$values = $data['values'];
foreach ($values as &$value) {
        switch ($value['type']) {
                default:
                        $value['value'] = true;
        }
}
unset($value);
echo var_dump($values)."\n".var_dump($data)."\n";

Expected result:
----------------
array(1) {
  [0]=>
  array(2) {
    ["type"]=>
    string(1) "4"
    ["value"]=>
    bool(true)
  }
}
array(1) {
  ["values"]=>
  array(1) {
    [0]=>
    array(1) {
      ["type"]=>
      string(1) "4"
    }
  }
}


Actual result:
--------------
array(1) {
  [0]=>
  array(2) {
    ["type"]=>
    &string(1) "4"
    ["value"]=>
    bool(true)
  }
}
array(1) {
  ["values"]=>
  array(1) {
    [0]=>
    array(1) {
      ["type"]=>
      &string(1) "4"
    }
  }
}