session register error problem

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HTML

PHP

deal all,

i have some problem when i try to log into the admin page i have received this kind of error

Fatal error: Call to undefined function session_is_registered() in /home/jobsbere/public_html/admin/index.php on line 17

PHP code is below and please give me the solution

session_start();

// The below checks restricts the user not to move back to the pages
// by unregistering the session variable control

if(@$_SESSION['user_info']['sess_status'] == "yes"){
session_unregister('user_info');
unset($user_info);
if(session_is_registered('message')){
session_unregister('message');
unset($message);
}
}

// unregistering the cp_message variable if the branching control was from change_pwd.php
if(session_is_registered('cp_message')){
session_unregister('cp_message');
unset($cp_message);
}

//Registering the message variables
session_register('message');
session_register('cp_message');

?>