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    <p>The problem is that assigning an array does not copy references, but makes a copy of the array. This means, that the "Second Child" array you have in "children" of the "Parent" node is not the same array that you add "Grandchild" to, but a copy of it.</p> <p>To resolve the issue, you have to explicitly use reference assignment instead of copying:</p> <pre><code>function projectListToTree($projects) { $stack = Array(); for($x =0; $x &lt; count($projects); $x++) { $project = &amp;$projects[$x]; $project['Children'] = array(); while(count($stack) &gt; 0 &amp;&amp; $stack[count($stack) - 1]['Right'] &lt; $project['Right']) { array_pop($stack); } if(count($stack) &gt; 0) { $stack[count($stack) - 1]['Children'][] = &amp;$project; echo "Adding " . $project['Name'] . " to " . $stack[count($stack) - 1]['Name'] . " for a total of " . count($stack[count($stack) - 1]['Children']) . " kids\n"; echo "\n"; } else { echo "No parent\n"; } echo "stack count: " . count($stack) . "\n"; $stack[] = &amp;$project; } echo "Left in stack: " . count($stack) . "\n"; return $stack[0]; } </code></pre> <p>Note, that an ampersand was added at three places.</p> <p>Because of this issue, one has to be extremely careful when using nested arrays and assignment operators in php.</p> <p>This also means that processor usage and memory footprint is intensive when using lots of data in nested arrays. For example, in the above example, when projectListToTree() returns, the complete array tree is copied to the local variable $tree and (since the php garbage collector sucks) is in memory twice.</p>
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    1. POHow to convert this MPTT array into a tree structure in PHP?
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    1. COThanks! I guess I'm still used to languages that treat arrays as objects and do assignments by reference. This worked perfectly, other than a warning about the array_push($stack, &$project) line (Call-time pass-by-reference has been deprecated), which I corrected by changing it to $stack[] = &$project;.
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    2. COI've changed the code in the answer to circumvent the warning.
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    3. COgreat solution, thanks !
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