Get the attachment ID for a given file url in WordPress

get_attachment_id_from_url.php

<?php

if ( ! function_exists( 'get_attachment_id_from_url' ) ) {
  /**
  * Get the attachment ID for a given file url
  *
  * @link   http://wordpress.stackexchange.com/a/7094
  * @param  string $url
  * @return boolean|integer
  */
  function get_attachment_id_from_url ($url) {
    $dir = wp_upload_dir();

    if (false === strpos($url, $dir['baseurl'] . '/')) {
      return false;
    }

    $file  = basename( $url );
    $query = array(
      'post_type'  => 'attachment',
      'fields'     => 'ids',
      'meta_query' => array(
        array(
          'key'     => '_wp_attached_file',
          'value'   => $file,
          'compare' => 'LIKE'
        )
      )
    );

    $ids = get_posts( $query );

    if (!empty($ids)) {
      foreach ($ids as $id) {
        if (wp_get_attachment_url($id) === $url) {
          return $id;
        }
      }
    }

    return false;
  }
}