What i always do is make an SQL table:

id - ipadres - userid - uniquestring
1 - 127.0.0.1 - 1 - bvt54345bw45herw45vw45wgef

And save a cookie named "login" or something containing the uniquestring. Then check the table for a record containing the uniquestring and ipadres. And then if its found login the user since you have it's userid.

Although i use this for autologin, you can also check it for login validation.

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Nunchaku! Who doesn't like martial arts? =)