Reply To: [Waiting for user confirmation] Featured Image from image field

// This code makes a "Featured Image" thumbnail from an image custom field
// Must be put into your themes functions.php file
// Must be used with FJarrets Function which can be found at:
// http://frankiejarrett.com/get-an-attachment-id-by-url-in-wordpress 
// ***** Change "wpcf-whoswho-main-image" to the name of the field you wish to pull from ******

function field_to_featured() {
	// Skip everything if thumbnail already exists
	$already_has_thumb = has_post_thumbnail($post->ID);
	if (!$already_has_thumb)  {
		global $post;
		//Get URL of image from the Custom Field
		$whos_imageurl=get_metadata($meta_type="post", $item_id=$post->ID, $meta_key="wpcf-whoswho-main-image", $single=true);
			// Skip if Custom Field is empty
			if ($whos_imageurl) {
				$attachment_id = fjarrett_get_attachment_id_by_url( $whos_imageurl ); //Use FJarrets Function to convert to an image ID number
				set_post_thumbnail($post->ID, $attachment_id); //Actually write the new ID number into the thumbnail field
				}
	}
}  //end function
// These add actions based on when you update a post. Works with Custom Post Types too. Any can be removed.
add_action('save_post', 'field_to_featured');
add_action('draft_to_publish', 'field_to_featured');
add_action('new_to_publish', 'field_to_featured');
add_action('pending_to_publish', 'field_to_featured');
add_action('future_to_publish', 'field_to_featured');
add_action('the_post', 'field_to_featured'); //This one is really agressive and adds a thumbnail just by looking at a post!

/* Get an attachment ID by URL in WordPress
 * http://frankiejarrett.com/get-an-attachment-id-by-url-in-wordpress
 * Return an ID of an attachment by searching the database with the file URL.
 *
 * First checks to see if the $url is pointing to a file that exists in
 * the wp-content directory. If so, then we search the database for a
 * partial match consisting of the remaining path AFTER the wp-content
 * directory. Finally, if a match is found the attachment ID will be
 * returned.
 *
 * @return {int} $attachment
 */
function fjarrett_get_attachment_id_by_url( $url ) {
 
	// Split the $url into two parts with the wp-content directory as the separator.
	$parse_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( $parse_url[1] ) || empty( $parse_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;", $parse_url[1] ) );
 
	// Returns null if no attachment is found.
	return $attachment[0];
}