functions.php

if( ! function_exists( 'get_attachment_id_from_url' ) ){

	function get_attachment_id_from_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 image
		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
			$attachment_url = preg_replace( '/-d+xd+(?=.(jpg|jpeg|png|gif)$)/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;
	}

}