Short example of getting customer_id of a Magento customer via email address outside of Magento via direct SQL

GetMagentoCustomerIDbyEmail.php

<?php
require_once('/site/app/Mage.php');
umask(0);
Mage::app();

//setup the direct sql db conn resource
$db_conn = Mage::getSingleton('core/resource');
$r_conn = $db_conn->getConnection('core_read');

$email = '[email protected]';

$sql = "SELECT entity_id FROM `customer_entity` WHERE email = '". $email . "' LIMIT 1";
$dat = $r_conn->fetchAll($sql);
$customer_id = $dat[0]['entity_id'];

echo $customer_id;
?>