just made a patch against the current sources (session.c and php_session.h).

*** php_session.h.orig  Tue May 15 15:16:50 2001
--- php_session.h       Tue May 15 15:23:26 2001
***************
*** 96,100 ****
--- 96,103 ----
        zend_bool define_sid;
        zend_bool use_cookies;
+       int readonly;
  } php_ps_globals;
+
+ #define SESS_READONLY 1

  extern zend_module_entry session_module_entry;
*** session.c.orig      Tue May 15 15:16:04 2001
--- session.c   Wed May 16 11:54:31 2001
***************
*** 526,529 ****
--- 526,533 ----
        PLS_FETCH();

+       if (PS(readonly)) {
+               return;
+       }
+
        if (!PG(register_globals)) {
                if (!PS(http_session_vars)) {
***************
*** 899,902 ****
--- 903,911 ----
        zend_bool retval = SUCCESS;

+       if (PS(readonly)) {
+               php_error(E_WARNING, "Trying to destroy readonly session");
+               return FAILURE;
+       }
+
        if (PS(nr_open_sessions) == 0) {
                php_error(E_WARNING, "Trying to destroy uninitialized session");
***************
*** 1265,1270 ****
--- 1274,1297 ----
  PHP_FUNCTION(session_start)
  {
+       pval **flag;
        PSLS_FETCH();

+       if (ZEND_NUM_ARGS() > 1)
+               WRONG_PARAM_COUNT;
+
+       if (ZEND_NUM_ARGS() == 0 ) {
+               PS(readonly) = 0;
+       }
+       if (ZEND_NUM_ARGS() == 1 && zend_get_parameters_ex(1, &flag) != FAILURE) {
+               convert_to_long_ex(flag);
+               if (((int) ((*flag)->value.lval)) == SESS_READONLY) {
+                       PS(readonly) = 1;
+               }
+               else {
+                       PS(readonly) = 0;
+               }
+       }
+
+
        php_session_start(PSLS_C);

***************
*** 1314,1317 ****
--- 1341,1347 ----
        PSLS_FETCH();

+       if (PS(readonly))
+               return;
+
        if (PS(nr_open_sessions) == 0)
                RETURN_FALSE;
***************
*** 1353,1356 ****
--- 1383,1388 ----
        PSLS_FETCH();

+       REGISTER_LONG_CONSTANT("SESS_READ_ONLY", SESS_READONLY, CONST_CS);
+
        php_rinit_session_globals(PSLS_C);

***************
*** 1404,1407 ****
--- 1436,1440 ----
        PS(module_number) = module_number;
        REGISTER_INI_ENTRIES();
+       REGISTER_LONG_CONSTANT("SESS_READ_ONLY", SESS_READONLY, CONST_CS);
        return SUCCESS;
  }

Forgot to include the batteries :)
After the patch above is applied, one could do:
session_start(SESS_READ_ONLY);
to start a readonly session. 
Functions that supposed to write the session data (core session functions, not actual savehandler functions) will be disabled.
On the other page, if session_start() is called without the  SESS_READ_ONLY flag, one could get the normal fully functional session, which will write the data. That would allow to use session in framed pages, when one frame is allowed to change the session data and another frames only read the data, and in many other cases. E.g. for me the feature has become inevitable when I needed to write a support chat, which should read session variables, but should not change them and, the most important, it should not save them, because a client could browse other parts of the site  (and this could affect the sesson vars) while he is chatting with the support. Without the readonly possibility, the new session variables could be easily rewrited by the chat script with outdated values.

seems fixed then. reopen if I am wrong

Why was this closed? I think it is an important feature req.

see: http://www.leaseweblabs.com/2014/08/session-locking-non-blocking-read-sessions-php/

You can read and close it immediately with session_write_close(), which frees the lock, but it's not quite the same.

For comparison, ASP.NET supports a ReadOnly flag which lets you specify that you will not write to the session:
http://msdn.microsoft.com/en-us/library/vstudio/16kf4xz0%28v=vs.100%29.aspx

This feature has been added in PHP 7:
session_start(['read_and_close'  => true])