Phpsession security issue analysis

By investing a lot of energy, attackers try to obtain valid session IDs of existing users. with session IDs, they may be able to have the same capabilities as this users in the system. therefore, our main solution is to verify the validity of the session ID.
Reference content is as follows:

The code is as follows:

If (! Isset ($ _ SESSION ['User _ agent']) {
$ _ SESSION ['User _ agent'] = $ _ SERVER ['remote _ ADDR ']. $ _ SERVER ['http _ USER_AGENT'];
}
/* If the user's session ID is forged */
Elseif ($ _ SESSION ['User _ agent']! = $ _ SERVER ['remote _ ADDR ']. $ _ SERVER ['http _ USER_AGENT']) {
Session_regenerate_id ();
}
?>