How to check user’s password?

WordPress does not just MD5 hash the password, it runs it through wp_hash_password() which in turn runs through the $wp_hasher (a global object) HashPassword method. This does some hsld crap to include getting random bytes of data, salting the password, and encrypting it. Basically it’s doing a whole lot more than an md5 to make it VERY difficult to reverse, even if you have the hashed password.