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    <p>The following code-example <em>converts</em> the array <code>$array</code> into the tree-structure you're looking for:</p> <pre><code>// key the array by id $keyed = array(); foreach($array as &amp;$value) { $keyed[$value['id']] = &amp;$value; } unset($value); $array = $keyed; unset($keyed); // tree it $tree = array(); foreach($array as &amp;$value) { if ($parent = $value['parent_id']) $array[$parent]['children'][] = &amp;$value; else $tree[] = &amp;$value; } unset($value); $array = $tree; unset($tree); var_dump($array); # your result </code></pre> <p>This does not work, if there is an <em>existing</em> parent id that is <code>0</code>. But could be easily changed to reflect that.</p> <p>This is a related question, that has the original array already keyed, so the first half of the solution could be spared: <a href="https://stackoverflow.com/questions/7673044/nested-array-third-level-is-disappearing/7673415">Nested array. Third level is disappearing</a>.</p> <p><strong>Edit:</strong></p> <p>So how does this work? This is making use of PHP variable aliasing (also known as <a href="http://www.php.net/manual/en/language.references.php" rel="nofollow noreferrer">references</a>) and (temporary) arrays that are used to store a) aliases to the nodes (<code>$keyed</code>) and b) to build the new tree order (<code>$tree</code>).</p> <blockquote> <p>Could you [...] explain the purpose of <code>$array = $keyed</code>, <code>$array = $tree</code> and the unsets?</p> </blockquote> <p>As both, <code>$keyed</code> and <code>$tree</code> contain references to values in <code>$array</code>, I first copy over that information into <code>$array</code>, e.g.:</p> <pre><code>$array = $keyed; </code></pre> <p>As now <code>$keyed</code> is still set (and contains references to the same values as in <code>$array</code>), <code>$keyed</code> is unset:</p> <pre><code>unset($keyed); </code></pre> <p>This un-sets all references in <code>$keyed</code> and ensures, that all values in <code>$array</code> aren't referenced any longer (the value's refcount is reduced by one).</p> <p>If the temporary arrays are not unset after the iteration, their references would still exist. If you use <code>var_dump</code> on <code>$array</code>, you would see that all values would have a <code>&amp;</code> in front, because they are still referenced. <code>unset($keyed)</code> removes these references, <code>var_dump($array)</code> again, and you will see the <code>&amp;</code>s are gone.</p> <p>I hope this was understandable, references can be hard to follow sometimes if you're not fluent with them. It often helps me to think about them as variable aliases.</p> <p>If you want some exercise, consider the following:</p> <blockquote> <p>How to convert your <code>$array</code> from flat to tree with one foreach iteration?</p> </blockquote> <p>Decide on your own when you would like to click the link which contains a <a href="http://codepad.viper-7.com/IUUDC4" rel="nofollow noreferrer">Solution</a>.</p>
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    1. POConverting an array from one to multi-dimensional based on parent ID values
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    1. COThat's amazing! Could you add comments to explain the purpose of `$array = $keyed`, `$array = $tree` and the unsets?
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    2. CO@Emanuil Rusev: I added a part about what the assignments and the unsets are about. And a little exercise if you like. Let me know; The solution of the exercise is actually a valid answer to your question as well, so it could be added to the answer for future reference later on.
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    3. CO@EmanuilRusev: In another question, [PHP variables: references or copies](http://stackoverflow.com/questions/7691510/php-variables-references-or-copies/7693307), I answered in more detail about arrays and references as well. Might be something interesting for you, so I leave it as a comment here.
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