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    copied!<p>This is not necessarily a job for recursion, this can be done easily by iteration.</p> <pre><code>&lt;?php $data = array( array('id' =&gt; 1001, 'ParentId' =&gt; NULL, 'Name' =&gt; 'Computing', 'Level' =&gt; 0), array('id' =&gt; 1002, 'ParentId' =&gt; NULL, 'Name' =&gt; 'Cassettes &amp; Vinyl', 'Level' =&gt; 0), array('id' =&gt; 1003, 'ParentId' =&gt; 1001, 'Name' =&gt; 'CD Players', 'Level' =&gt; 1), array('id' =&gt; 1004, 'ParentId' =&gt; 1002, 'Name' =&gt; 'CD Writers', 'Level' =&gt; 1), array('id' =&gt; 1005, 'ParentId' =&gt; 1003, 'Name' =&gt; 'CD-ROM Drives', 'Level' =&gt; 2), array('id' =&gt; 1006, 'ParentId' =&gt; 1004, 'Name' =&gt; 'Computing', 'Level' =&gt; 3) ); $index = array(); $tree = array(); // step 1: build index (note that I use &amp;$row references!) foreach ($data as &amp;$row) { $index[$row['id']] = &amp;$row; $row['children'] = array(); if ($row['ParentId'] == NULL) { $tree[] = &amp;$row; } } // step 2: link tree (references here as well!) foreach ($data as &amp;$row) { $index[$row['ParentId']]['children'][] = &amp;$row; } print_r($tree); ?&gt; </code></pre> <p>result:</p> <pre><code>Array ( [0] =&gt; Array ( [id] =&gt; 1001 [ParentId] =&gt; [Name] =&gt; Computing [Level] =&gt; 0 [children] =&gt; Array ( [0] =&gt; Array ( [id] =&gt; 1003 [ParentId] =&gt; 1001 [Name] =&gt; CD Players [Level] =&gt; 1 [children] =&gt; Array ( [0] =&gt; Array ( [id] =&gt; 1005 [ParentId] =&gt; 1003 [Name] =&gt; CD-ROM Drives [Level] =&gt; 2 [children] =&gt; Array ( ) ) ) ) ) ) [1] =&gt; Array ( [id] =&gt; 1002 [ParentId] =&gt; [Name] =&gt; Cassettes &amp; Vinyl [Level] =&gt; 0 [children] =&gt; Array ( [0] =&gt; Array ( [id] =&gt; 1004 [ParentId] =&gt; 1002 [Name] =&gt; CD Writers [Level] =&gt; 1 [children] =&gt; Array ( [0] =&gt; Array ( [id] =&gt; 1006 [ParentId] =&gt; 1004 [Name] =&gt; Computing [Level] =&gt; 3 [children] =&gt; Array ( ) ) ) ) ) ) ) </code></pre>
 

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