Description:
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'foreach' fails to continue visiting the following items in the array after returning from a recursion function call. I understand that 'foreach' works on the copy of the array, but I think it cannot explain the situation. Because after returning from the recursion call, the memory space before the call should be popped out from the running enviorenment stack. Hence, foreach should not feel that its execution has been interrupted by the recursion call.
If I change the structure of the array slightly and use for loop to replace foreach the function get the expected value. 
Following is the revised version:
function recurGetDescdendant($all, $current, $descendant) {
 $length = count($all);
 for ($i = 0; $i < $length; $i++) {
 	$userid = $all[$i][0];
 	$parent = $all[$i][1];
 	if ($parent == $current) {
    $descendant[] = $userid;
    $cdescendant = array();
    recurGetDescdendant(&$all, $userid, &$cdescendant);
    $descendant = array_merge($descendant, $cdescendant);
  }  }  }
//key represents the user id; value reprents the parent's user id of this user. for example: 6 => 1 means user 6's parent is user 1
  $all = array(array(1,0), array(2,0), array(3,0), array(4,0), array(5,0), array(6,1), array(7,1), array(11,7), array(12,11), array(13,11) );
  $descendant = array();
  recurGetDescdendant(&$all, 1, &$descendant);
  foreach ($descendant as $value) echo $value . "<br>\n";

Reproduce code:
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function recurGetDescdendant($all, $current, $descendant) {
 foreach ($all as $userid => $parent) {
   if ($parent == $current) {
    $descendant[] = $userid;
    $cdescendant = array();
    recurGetDescdendant(&$all, $userid, &$cdescendant);
    $descendant = array_merge($descendant, $cdescendant);
  }  }  }
//key represents the user id; value reprents the parent's user id of this user. for example: 6 => 1 means user 6's parent is user 1
  $all = array(1 => 0, 2 => 0, 3 => 0, 4 => 0, 5 => 0, 6 => 1, 7 => 1, 11 => 7, 12 => 11, 13 => 11 );
  $descendant = array();
  recurGetDescdendant(&$all, 1, &$descendant);
  foreach ($descendant as $value) echo $value . "<br>\n";

Expected result:
----------------
6<br>
7<br>
11<br>
12<br>
13<br>

Actual result:
--------------
6<br>