Retrieve the purchase ID based on the purchase key

edd_get_purchase_id_by_key.php

<?php
/**
 * Retrieve the purchase ID based on the purchase key
 * 
 * @access		public
 * @since 		1.2.3
 *
 * @param 		string $key the purchase key to search for
 * @return 		int $order_id
 */
function edd_get_purchase_id_by_key( $key ) {
	$meta_query = array(
		array(
			'key' => '_edd_payment_purchase_key',
			'value' => $key
		),
	);

	$payments = get_posts( array( 'meta_query' => $meta_query, 'post_type' => 'edd_payment' ) );
	
	if ( count( $payments ) == 1 )
		return $payments[0]->ID;
		
	return 0;
}