<?php $age=array("Peter"=>"35","Ben"=>"37","Joe"=>"43"); foreach($age as $x=>$x_value) { echo "Key=" . $x . ", Value=" . $x_value; echo "<br>"; } ?>
$id = 1; $age = 2; $sort = "id"; // or "age"; $Key = $$sort; $arr = array($Key => 'string'); print_r($arr);