$arr = array (1, 2, 3, 4);
$total = count($arr) - 1;
$nestedarray = array();
for($i = $total; $i >= 0; $i--) {
$nestedarray = array($arr[$i] => $nestedarray);
}
print_r($nestedarray);
/*
run:
Array
(
[1] => Array
(
[2] => Array
(
[3] => Array
(
[4] => Array
(
)
)
)
)
)
*/