why regenerate session when use session_name function

Question

I use a php class for manage session on my own script. in this class i use one static method and on this static method i use session_name("some_name") for rename session default name "PHPSESSID". but when use session_name() function in every reload page session regenerate.

now why regenerate session when use session_name function?

class T_session
{

    // cookie new name
    // expire , path , domain , secure , http only
    static public function set_cookie($cookie_name , $expire = 0 , $path = "/" , $domain = null , $secure = null , $http_only = null ){

        // Set cookie name
        session_name($cookie_name . "_Session");

        // Set the domain to default to the current domain.
                $domain = isset($domain) ? $domain : Tbmedia_domain_root();

        // Set the default secure value to whether the site is being accessed with SSL
            $secure = isset($secure) ? $secure : Tbmedia_check_https();

            session_set_cookie_params($expire,$path,$domain,$secure,true);


            // Configuration php ini
           ini_set('session.save_handler','files');
           ini_set('session.use_cookies',1);
           ini_set('session.cookie_secure',1);
           ini_set('session.use_only_cookies',1);
           ini_set('session.cookie_domain',$domain);
           ini_set('session.cookie_httponly',1);
           ini_set('session.entropy_length',32);
           ini_set('session.entropy_file','/dev/urandom');
           ini_set('session.hash_function','sha256');
           ini_set('session.hash_bits_per_character',5);

           session_start();

    }
   }