<?php

$keystr = 'plant,fruit,exotic';
$value='kiwi'; 

$arr = array();
$current = &$arr;
$keys = explode(',', $keystr);
foreach($keys as $key) {
    $current[$key] = array();
    $current = &$current[$key];
}
$current = $value;
unset($current);
var_dump($arr);