$arr = array(4, 9, 12, 90, 13, 0, 3, 97);
$index = 4;
$elementvalue = $arr[$index];
unset($arr[$index]);
$arr[count($arr) + 1] = $elementvalue;
// reindex array
$arr = array_values($arr);
print_r($arr);
/*
run:
Array
(
[0] => 4
[1] => 9
[2] => 12
[3] => 90
[4] => 0
[5] => 3
[6] => 97
[7] => 13
)
*/