Function to retrieve a product SKU based on it's ID on Magento (1.8) platform.

getSkuById.php

<?php

include_once '../app/Mage.php';

/**
 * @author odahcam
 *
 * @version 1.0.0
 *
 * @param {string|int} $product_id
 *
 * @return {string}
 **/
function getSkuById($product_id)
{
    $adapter = Mage::getSingleton('core/resource')->getConnection('core_read');

    $select = $adapter
        ->select()
        ->from(Mage::getResourceModel('catalog/product')->getEntityTable(), 'sku')
        ->where('entity_id = :entity_id');

    return $adapter->fetchOne($select, [':entity_id' => (string) $product_id]);
}