Magento function to fetch Magento product SKU by ID

getSkuById.php

<?php
function getSkuById($productId)
{
  $productResourceModel = Mage::getResourceModel('catalog/product');
  $adapter = Mage::getSingleton('core/resource')->getConnection('core_read');
  $select = $adapter->select()
    ->from($productResourceModel->getEntityTable(), 'sku')
    ->where('entity_id = :entity_id');
  $bind = array(':entity_id' => (string)$productId);
  $productSku = $adapter->fetchOne($select, $bind);
  return $productSku;
}