how to assign key for array element in php ? code example

Example 1: php variables as keys in arrays

$id = 1;
$age = 2;    
$sort = "id"; // or "age";
    $Key = $$sort;
$arr = array($Key => 'string');

print_r($arr);

Example 2: php array push key value

<?php
$image[0] = $image[0].','.$filename;
?>