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run code in 150+ php & hhvm versions
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<?php $arr = array(); $arr[1] = array('p' => 3, 'id' => 4); $arr[2] = array('p' => 2, 'id' => 3); $arr[3] = array('p' => 1, 'id' => 2); $arr[4] = array('p' => 0, 'id' => 1); $mas = array(); foreach($arr as $ind => $itm) { $mas[$itm['p']][$itm['id']] = 'aaa'.$ind; } print_r($mas);
Output for 4.3.0 - 7.1.0
Array ( [3] => Array ( [4] => aaa1 ) [2] => Array ( [3] => aaa2 ) [1] => Array ( [2] => aaa3 ) [0] => Array ( [1] => aaa4 ) )