The decision to enable the rewriter is made before your script starts running.
Once it has started it cannot be turned off again.

Why is that so? In my eyes, the point that makes sense for the rewriter to be started, is session_start().

Furthermore: If I kind of 'deactivate' the rewriter by ini_set ('url_rewriter.tags', ''); it also has effect on links that were already sent to output:

---------------------------------------
<?php

session_start ();

?>
<a href="a.php">Test</a>
<?php
ini_set ('url_rewriter.tags', '');
?>
<a href="a.php">Test</a>
---------------------------------------

will generate:

<a href="a.php">Test</a>
<a href="a.php">Test</a>

whereas 

---------------------------------------
<?php

session_start ();

?>
<a href="a.php">Test</a>
<?php
// ini_set ('url_rewriter.tags', '');
?>
<a href="a.php">Test</a>
---------------------------------------

appends the session id to both links.

Therefore I don't quite understand why deactivating the rewriter before session_start() is not possible whereas modifying it is even possible after session_start() and even after output that has to be rewritten was already generated.

I tried to figure out myself how this could be solved. I just changed the definition of session.use_trans_sid to PHP_INI_ALL and I recompiled my PHP module.

Now, I can change session.use_trans_sid before session_start() is called and it works fine.

Here's the 'patch' (against PHP 4.3.2):

---------------------------------------------------------
--- ext/session/session.c.old	2003-07-19 12:29:58.000000000 +0200
+++ ext/session/session.c	2003-07-19 12:34:31.000000000 +0200
@@ -151,7 +151,7 @@
 	STD_PHP_INI_ENTRY("session.entropy_length",     "0",         PHP_INI_ALL, OnUpdateInt,    entropy_length,     php_ps_globals,    ps_globals)
 	STD_PHP_INI_ENTRY("session.cache_limiter",      "nocache",   PHP_INI_ALL, OnUpdateString, cache_limiter,      php_ps_globals,    ps_globals)
 	STD_PHP_INI_ENTRY("session.cache_expire",       "180",       PHP_INI_ALL, OnUpdateInt,    cache_expire,       php_ps_globals,    ps_globals)
-	STD_PHP_INI_BOOLEAN("session.use_trans_sid",    "0",         PHP_INI_SYSTEM|PHP_INI_PERDIR, OnUpdateBool,   use_trans_sid,      php_ps_globals,    ps_globals)
+	STD_PHP_INI_BOOLEAN("session.use_trans_sid",    "0",         PHP_INI_ALL, OnUpdateBool,   use_trans_sid,      php_ps_globals,    ps_globals)
 
 	/* Commented out until future discussion */
 	/* PHP_INI_ENTRY("session.encode_sources", "globals,track", PHP_INI_ALL, NULL) */
---------------------------------------------------------

see  http://bugs.php.net/bug.php?id=14991 
- this wont really fix it

I agree it's a feature request - it would have to be a function like session_use_transid(true|false) as there is no mechanism for ini_set to check whether output has started...

> as there is no mechanism for ini_set to check whether output has started

There is a mechanism: these OnUpdate*-functions. I added a function OnUpdateTransSid which is essentially the OnUpdateBool function definied within Zend/zend_ini.c but with an if-block in front of it, if the session was already started. (the if-block was taken from another OnUpdate-function that was in session.c)

It probably would be cleaner to call OnUpdateBool directly instead of just copying the code, but as all those functions are defined by precompiler macros, I assume the parameters of these functios could change, so just copying the code from OnUpdateBool seems to be more portable to me.

Here are my changes so far: (tested with PHP 4.3.2)

-----------------------------------------------------------------------
--- ext/session/session.c.old	2003-07-19 12:29:58.000000000 +0200
+++ ext/session/session.c	2003-07-19 13:46:33.000000000 +0200
@@ -105,6 +105,32 @@
 	return SUCCESS;
 }
 
+static PHP_INI_MH(OnUpdateTransSid)
+{
+	if (PS(session_status) == php_session_active) {
+		php_error_docref(NULL TSRMLS_CC, E_WARNING, "A session is active. You cannot change the session module's ini settings at this time.");
+		return FAILURE;
+	}
+
+	zend_bool *p;
+#ifndef ZTS
+	char *base = (char *) mh_arg2;
+#else
+	char *base;
+
+	base = (char *) ts_resource(*((int *) mh_arg2));
+#endif
+
+	p = (zend_bool *) (base+(size_t) mh_arg1);
+
+	if (strncasecmp("on", new_value, sizeof("on"))) {
+		*p = (zend_bool) atoi(new_value);
+	} else {
+		*p = (zend_bool) 1;
+	}
+	return SUCCESS;
+}
+
 static PHP_INI_MH(OnUpdateSerializer)
 {
 	if (PS(session_status) == php_session_active) {
@@ -151,7 +177,7 @@
 	STD_PHP_INI_ENTRY("session.entropy_length",     "0",         PHP_INI_ALL, OnUpdateInt,    entropy_length,     php_ps_globals,    ps_globals)
 	STD_PHP_INI_ENTRY("session.cache_limiter",      "nocache",   PHP_INI_ALL, OnUpdateString, cache_limiter,      php_ps_globals,    ps_globals)
 	STD_PHP_INI_ENTRY("session.cache_expire",       "180",       PHP_INI_ALL, OnUpdateInt,    cache_expire,       php_ps_globals,    ps_globals)
-	STD_PHP_INI_BOOLEAN("session.use_trans_sid",    "0",         PHP_INI_SYSTEM|PHP_INI_PERDIR, OnUpdateBool,   use_trans_sid,      php_ps_globals,    ps_globals)
+	STD_PHP_INI_BOOLEAN("session.use_trans_sid",    "0",         PHP_INI_ALL, OnUpdateTransSid, use_trans_sid,      php_ps_globals,    ps_globals)
 
 	/* Commented out until future discussion */
 	/* PHP_INI_ENTRY("session.encode_sources", "globals,track", PHP_INI_ALL, NULL) */
-----------------------------------------------------------------------

This bug has been fixed in CVS.

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