1.set the different value by session_name() to the php.ini
(normal when the same value)
2.execute session_regenerate_id();
3.set some session value.
4.at new request,the value will lose

Apparently, you didn't call session_start()

session auto start.

the new name(cd) session tmp file,content is 
a|i:1;
the ab session tmp file ,content is 
//(null)

right value.

so i looked http://php.net/manual/zh/function.session-name.php
said
----
Thus, you need to call session_name() for every request (and before session_start() or session_register() are called). 
----

so i turn auto start off,
and run
session_name('cd');
session_start();//$_SESSION=['a'=>1];
session_name('ab');//$_SESSION=[];
session_start();//$_SESSION=[];

it's ok.

Exactly. You can change the session name in code but it's only temporary - PHP won't remember it for next time. You have to change it every time, and it has to be done before PHP tries to access the session data.

Your problem is that you have more than one call to session_start(), and the second call fails just like the documentation says it should.

If you have already used session_start() then you cannot call it again with calling session_write_close() beforehand. Your code should be as follows: 

session_start();
session_name('newname');
session_regenerate_id();
session_write_close();
session_start();