How can I integrate the PHP API into my website. Where I get the Documentation. Please help me
I do believe you need some programing skills to have CM fully integrated into your existing website.
If you are reselling CM to your clients:
If you are not a skilled coder, and have no budget to get help from experts, then I will suggest to have the client creation done offline (provide a sign up form in your website, create client manually upon receive of request), and make a sign in (log in) form in your existing website to allow clients log in to the CM control panel.