Online PHP editor | output for 4JuIa

<?php

$comments = [
    1 => ['content' => "The first comment", 'parent' => 0],
    2 => ['content' => "The second comment", 'parent' => 0],
    3 => ['content' => "Reply of first comment", 'parent' => 1],
    4 => ['content' => "Another reply of first comemnt", 'parent' => 1],
    5 => ['content' => "Reply of a reply of the first comment", 'parent' => 4]
];


foreach($comments as $id => &$value)
{
    # check if there is a parent
    if ($parentId = $value['parent'])
    {
        $comments[$parentId][0][$id] =& $value; # add child to parent
        unset($comments[$id]); # remove reference from topmost level
    }
}
unset($value); # remove iterator reference
print_r($comments); # your tree

// foreach($comments as $id => &$c) {
//     if (isset($comments[$c['parent']])) {
//         $comments[$c['parent']]['comments'][$id] =& $c;
//         unset($comments[$id]);
//     }
// }

// var_dump($comments);

eol versions

based on Ck1VH

Output for 7.0.0 - 7.2.0: Notice: Undefined index: parent in /in/4JuIa on line 15 Array ( [1] => Array ( [content] => The first comment [parent] => 0 [0] => Array ( [3] => Array ( [content] => Reply of first comment [parent] => 1 ) [4] => Array ( [content] => Another reply of first comemnt [parent] => 1 ) ) ) [2] => Array ( [content] => The second comment [parent] => 0 ) [4] => Array ( [0] => Array ( [5] => Array ( [content] => Reply of a reply of the first comment [parent] => 4 ) ) ) )
Output for 5.4.0 - 5.6.28: Array ( [1] => Array ( [content] => The first comment [parent] => 0 [0] => Array ( [3] => Array ( [content] => Reply of first comment [parent] => 1 ) [4] => Array ( [content] => Another reply of first comemnt [parent] => 1 ) ) ) [2] => Array ( [content] => The second comment [parent] => 0 ) [4] => Array ( [0] => Array ( [5] => Array ( [content] => Reply of a reply of the first comment [parent] => 4 ) ) ) )
Output for 5.3.18 - 5.3.29: Parse error: syntax error, unexpected '[' in /in/4JuIa on line 3
Process exited with code 255.