Get a WordPress attachment ID given its URL

functions.php

<?php

/**
 * Get attachment ID from its URL
 *
 * @param string $url
 * @return bool|int The Attachment ID or false if not found
 */
function get_attachment_id_from_src($url) {
	if (($pos = strpos($url, '/uploads/')) === false) {
		// URL does not contain /uploads/, so we won't be able to find an ID
		return false;
	}

	// Drop everything up to (and including) /uploads/ in the URL
	// e.g. "http://example.com/wp-content/uploads/2017/06/file.pdf"
	//      => "2017/06/file.pdf"
	$url_part = substr($url, $pos + strlen('/uploads/'));

	$query = new WP_Query([
		'post_type' => 'attachment',
		'post_status' => 'inherit',
		'meta_key' => '_wp_attached_file',
		'meta_value' => $url_part,
		'posts_per_page' => 1,
		'fields' => 'ids',
	]);

	return ($query->post_count > 0) ? (int) $query->posts[0] : false;
}