Do I seek permission from Facebook,Twitter,Yahoo to do this (Please view details)?

[AdamT]

I have a website, its members (my site's users) like to signin my site with their facebook, twitter, yahoo ID & password.

Can I do it with PHP? If so, there is any links for reference/samples please?

There is any legal matters involved here?

Do I need to get permission (written or in any other ways) to do add that feature?

waiting for your details.

thanks in advance

[just "JR"]

No legalities involved in this one...
You "just need" to use OAuth or OpenID systems and the relative SN's APIs.
BEWARE: it is DEFINITELY NOT for beginners!
You need a very solid experience in Php and the use of API to even get close...
Google OAuth and OpenID. Masses of details, masses of scripts, masses of documentation, masses of complications...
OAuth is free, while OpenID requires a paid subscription.

(With 18 years experience in web development, it took me three weeks to get through...)
(I now offer it to my clients, but the fee is 1000€ for the set-up, while I can make you a full e-commerce for the same price!).