How do I enable single sign in to WordPress backend from my already existing website backend?

I want to be able to directly enter my WordPress blog as the WordPress admin once I have authenticated to the back end of my current PHP site as the admin of that site. Is there a WordPress API, cookies or whatever, I can utilize to pass sign-in credentials from my current PHP site so I don’t have to sign-in twice every time I want to post a blog while I am authenticated on my current site?