Get the ID of an attachment image using the file URL. Original: https://frankiejarrett.com/2013/05/get-an-attachment-id-by-url-in-wordpress/

wp_url_to_attachmentid.php

<?php
/**
 *  Get the ID of an attachment image using the file URL.
 *
 *  @param   string  $url         The URL of the image
 *  @return  int     $attachment  The attachment image ID
 */
function wp_url_to_attachmentid( $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];
}