I am developing an extension for the cartthrob addon.
I'd like to get the url of checkout page in ext.myaddon.php. How can I get this?
There's no specific "checkout page" in CartThrob. A checkout form can be placed on any -- and as many -- templates as the developer would like. Therefore the best thing would be to determine what page the site owner wants to direct them to and let them set it in your extension settings or a tag parameter.