Login to wordpress from another PHP site

This is feeling quite difficulty for me. Please help.
I’ve a website created with cake PHP in root domain (say mysite.com) and a blog in a directory called “blog” (mysite.com/blog).
I have a register/login feature in the PHP site. So what I want here is, whenever a member login to the main site using the PHP login form, and after he open the blog (mysite.com/blog), he should logged-in as the same user logged in the main PHP site. In the same way, whenever a new user sign-up with the registration form in the main PHP site, he should become an user in the wordpress blog.
At this moment, please help me in the case of login only.
Thanks.