<?php

function nested_array($arr, $i, $size)
{
    if ($i == ($size-1))
    {
        return array($arr[$i] => ".");
    }

    return array($arr[$i] => nested_array($arr,($i+1),$size));
}

print_r(nested_array(array("I", "need", "this", "to", "be", "nested"),0,6));