gistfile1.php

<?php
/**
 * Return an ID of an attachment by searching the database with the file URL.
 *
 * First checks to see if the $url is pointing to a file that exists in
 * the wp-content directory. If so, then we search the database for a
 * partial match consisting of the remaining path AFTER the wp-content
 * directory. Finally, if a match is found the attachment ID will be
 * returned.
 *
 * @param string $url The URL of the image (ex: http://mysite.com/wp-content/uploads/2013/05/test-image.jpg)
 * 
 * @return int|null $attachment Returns an attachment ID, or null if no attachment is found
 */
function fjarrett_get_attachment_id_by_url( $url ) {
	// Split the $url into two parts with the wp-content directory as the separator
	$parsed_url  = explode( parse_url( WP_CONTENT_URL, PHP_URL_PATH ), $url );

	// Get the host of the current site and the host of the $url, ignoring www
	$this_host = str_ireplace( 'www.', '', parse_url( home_url(), PHP_URL_HOST ) );
	$file_host = str_ireplace( 'www.', '', parse_url( $url, PHP_URL_HOST ) );

	// Return nothing if there aren't any $url parts or if the current host and $url host do not match
	if ( ! isset( $parsed_url[1] ) || empty( $parsed_url[1] ) || ( $this_host != $file_host ) ) {
		return;
	}

	// Now we're going to quickly search the DB for any attachment GUID with a partial path match
	// Example: /uploads/2013/05/test-image.jpg
	global $wpdb;

	$attachment = $wpdb->get_col( $wpdb->prepare( "SELECT ID FROM {$wpdb->prefix}posts WHERE guid RLIKE %s;", $parsed_url[1] ) );

	// Returns null if no attachment is found
	return $attachment[0];
}