Retrieve the URL for an attachment.

Signature

wp_get_attachment_url( $post_id = 0 )
post_id
 (int) Attachment ID.
Default: 0

Return

(string)

Source

function wp_get_attachment_url( $post_id = 0 ) {
	$post_id = (int) $post_id;
	if ( !$post = get_post( $post_id ) )
		return false;

	if ( 'attachment' != $post->post_type )
		return false;

	$url = '';
	if ( $file = get_post_meta( $post->ID, '_wp_attached_file', true) ) { //Get attached file
		if ( ($uploads = wp_upload_dir()) && false === $uploads['error'] ) { //Get upload directory
			if ( 0 === strpos($file, $uploads['basedir']) ) //Check that the upload base exists in the file location
				$url = str_replace($uploads['basedir'], $uploads['baseurl'], $file); //replace file location with url location
			elseif ( false !== strpos($file, 'wp-content/uploads') )
				$url = $uploads['baseurl'] . substr( $file, strpos($file, 'wp-content/uploads') + 18 );
			else
				$url = $uploads['baseurl'] . "/$file"; //Its a newly uploaded file, therefor $file is relative to the basedir.
		}
	}

	if ( empty($url) ) //If any of the above options failed, Fallback on the GUID as used pre-2.7, not recommended to rely upon this.
		$url = get_the_guid( $post->ID );

	$url = apply_filters( 'wp_get_attachment_url', $url, $post->ID );

	if ( empty( $url ) )
		return false;

	return $url;
}
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