Is there a simple method of getting the SiteID when I only know the site URL using an PHP API ?
Why do you not know the siteid? That should be something you know, look in websites under settings. I am pretty sure the API does not allow you to search for a siteid.
Yes it does, http://piwik.org/docs/analytics-api/reference/#SitesManager
Use SitesManager.getSitesIdFromSiteUrl (url)