Session Hijacking is term where attackers hold of a session identifier and is able to send requests as if they were that user. In particular, it is used to refer to the theft of a magic cookie used to authenticate a user to a remote server. It has particular relevance to web developers, as the HTTP cookies used to maintain a session on many web sites can be easily stolen by an attacker using an intermediary computer or with access to the saved cookies on the victim's computer (see HTTP cookie theft).
How to prevent your data from Session Hijacking
1) In php.ini set session.hash_function = sha256 or session.hash_function = sha512.
2) In php.ini set session.hash_bits_per_character = 5
3) Add "user agent" (browser) in session & check each subsequent request.
4) Add IP Address in session & check each subsequent request.
5) Change the name of the session from the default PHPSESSID
6) In secure pages ask for reenter the password.