You can convert an attachment url to its absolute path in WordPress using this function.

wp-attachment-url-to-path.php

	<?php

	/**
	 * Get the attachment absolute path from its url
	 *
	 * @param string $url the attachment url to get its absolute path
	 *
	 * @return bool|string It returns the absolute path of an attachment
	 */
	function attachment_url_to_path( $url )
	{
		$parsed_url = parse_url( $url );
		if(empty($parsed_url['path'])) return false;
		$file = ABSPATH . ltrim( $parsed_url['path'], '/');
		if (file_exists( $file)) return $file;
		return false;
	}