How to return array from recursive function

Question

I couldn't find solution for this and I don't have much time for this. So what I want is to make function that I give category ID and it returns all ID's of categories which are it's child categories.

function getID($var) {
 $categories = array();
 function getChildren($id) {
 $result = mysql_query("SELECT * FROM categories WHERE parentID = '$id'");
 echo "<ul>";
 while ($row = mysql_fetch_array($result)) {
 echo "<li><a>{$row['ID']}</a>";
 $categories[] = $row['ID'];
 getChildren($row['ID']);
 echo "</li>";
 }
 echo "</ul>";
 }
 getChildren($var);
 return $categories;
}

I wan't to store everything in $categories array. $var is category ID which I give to function. When I call this function it prints list of exactly what I want't but array is empty.

• Vimalnath: it prints list of exactly what I want't but array is empty it prints and its empty!?!?
• LaKaede: echo "<li><a>{$row['ID']}</a>" this line outputs ID what I want but when I try to print_r($categories) it doesn't output anything.
• Gerald Schneider: Do you want to return a flat list or a tree that represents the children?
• LaKaede: I want just flat list.
• Gerald Schneider: Then fredriks answer should do the trick

Answer 1

It seems you have a scope problem. Try this:

function getChildren(&$categories, $id) {
 $result = mysql_query("SELECT * FROM categories WHERE parentID = '$id'");
 echo "<ul>";
 while ($row = mysql_fetch_array($result)) {
 echo "<li><a>{$row['ID']}</a>";
 $categories[] = $row['ID'];
 getChildren($categories, $row['ID']);
 echo "</li>";
 }
 echo "</ul>";
}
function getID($var) {
 $categories = array();
 getChildren($categories, $var);
 return $categories;
}

Here is the PHP reference page describing how to pass by reference instead of by value. Basically it says that any function parameter which has a & in front of it will be passed by reference instead of by value.

• Gerald Schneider: Same problem as with Naryls answer, if you do it this way you have to pass a reference to the original array (as fabs answer suggests)
• LaKaede: oh, thanks. It works! Only one thing getChildren($categories, $var); should be $this->getChildren($categories, $var)
• fredrik: Oh, you have it in a class - that was not in the question. If course then you need the $this-> as well.

Answer 2

$categories only exists in the scope of get children. You could either pass it by reference as second parameter or create get children as a closure with use (&$categories)

Answer 3

function getChildren($id, $categories)

should work.

The array was out of scope, so the function didn't touch the one you are returning. It created a new local $categories inside the function.

• Gerald Schneider: This will return an empty array as well. The function getChildren() will work with a copy of the empty array, the original array will not be touched. If you want the function to add to the original array you have tu use function getChildren($id, &$categories);.