PHP Session handling class

I've written a custom PHP session class for handling sessions across the web app. Please review the code and point out mistakes and suggest better handling techniques.

require_once('config.php');

class Sessions {

protected $sessionID;

public function __construct(){
    if( !isset($_SESSION) ){
        $this->init_session();
    }
    //session_start();
    //$this->sessionID = session_id();
}

public function init_session(){
    session_start();
}

public function set_session_id(){
    //$this->start_session();
    $this->sessionID = session_id();
}

public function get_session_id(){
    return $this->sessionID;
}

public function session_exist( $session_name ){
    if( isset($_SESSION[$session_name]) ){
        return true;
    }
    else{
        return false;
    }
}

public function create_session( $session_name , $is_array = false ){
    if( !isset($_SESSION[$session_name])  ){
        if( $is_array == true ){
            $_SESSION[$session_name] = array();
        }
        else{
            $_SESSION[$session_name] = '';
        }
    }
}

public function insert_value( $session_name , array $data ){
    if( is_array($_SESSION[$session_name]) ){
        array_push( $_SESSION[$session_name], $data );
    }
}

public function display_session( $session_name ){
    echo '';print_r($_SESSION[$session_name]);echo '';
}

public function remove_session( $session_name = '' ){
    if( !empty($session_name) ){
        unset( $_SESSION[$session_name] );
    }
    else{
        unset($_SESSION);
        //session_unset();
        //session_destroy();
    }
}

public function get_session_data( $session_name ){
    return $_SESSION[$session_name];
}

public function set_session_data( $session_name , $data ){
    $_SESSION[$session_name] = $data;
}

}

Commented out Code

You should remove code that is commented out. If you think that you might need it in the future, think about using version control.

set_session_id()

This is only called in your commented out code. Does the user have to call it manually? If they don't, get_session_id() will return a wrong result. Maybe rewrite it like this (you don't seem to be using the field $sessionID, so you might as well get rid of it):

public function get_session_id(){
    return session_id();
}

if-else and session_exist

If you have code like this:

if( isset($_SESSION[$session_name]) ){
    return true;
}
else{
    return false;
}

you can rewrite it like this:

return isset($_SESSION[$session_name]);

Also, when you define a function like this, then use it. Instead of

if( !isset($_SESSION[$session_name])  ){

write

if(!session_exist($session_name]) ){

XSS

Using Session for XSS might be a possibility, depending on how your code is used. So in display_session I would clean the session with htmlentities to prevent XSS attacks.

StackExchange Code Review Q#61066, answer score: 9