get_id_from_attachment_url.php

<?php

/**
 * Retrive ID from attachment URL
 * Source http://philipnewcomer.net/2012/11/get-the-attachment-id-from-an-image-url-in-wordpress/
 */

function get_id_from_attachment_url($attachment_url = '') {
  
  global $wpdb;
  
  $attachment_id = false;
  
  // If there is no url, return.
  if ( '' == $attachment_url ) return;
  
  // Get the upload directory paths
  $upload_dir_paths = wp_upload_dir();
  
  // Make sure the upload path base directory exists in the attachment URL, to verify that we're working with a media library imag
  if ( false !== strpos( $attachment_url, $upload_dir_paths['baseurl'] ) ) {
    
    // If this is the URL of an auto-generated thumbnail, get the URL of the original image
    $attachment_url = preg_replace( '/-\d+x\d+(?=\.(jpg|jpeg|png|gif)$)/i', '', $attachment_url );
    
    // Remove the upload path base directory from the attachment URL
    $attachment_url = str_replace( $upload_dir_paths['baseurl'] . '/', '', $attachment_url );
    
    // Finally, run a custom database query to get the attachment ID from the modified attachment URL
    $attachment_id = $wpdb->get_var( $wpdb->prepare( "SELECT wposts.ID FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta WHERE wposts.ID = wpostmeta.post_id AND wpostmeta.meta_key = '_wp_attached_file' AND wpostmeta.meta_value = '%s' AND wposts.post_type = 'attachment'", $attachment_url ) );
  }
  return $attachment_id;
}