Description:
------------
In HEAD, the original input is sent to the output when the output buffer callback returns NULL. The docs for ob_start() only state this should happen when it returns FALSE - not NULL:

"If output_callback  returns FALSE original input is sent to the browser."

On PHP 5_2 and 5_3, the empty string is sent to the output when the output buffer callback returns NULL.

If the 5_2 & 5_3 behaviour is correct, here is a patch for HEAD :

Index: output.c
===================================================================
RCS file: /repository/php-src/main/output.c,v
retrieving revision 1.214
diff -u -w -p -r1.214 output.c
--- output.c	18 Aug 2008 07:45:59 -0000	1.214
+++ output.c	18 Dec 2008 14:08:53 -0000
@@ -983,7 +983,7 @@ static inline php_output_handler_status_
 			ZVAL_LONG(ob_mode, (long) context->op);
 			zend_fcall_info_argn(&handler->func.user->fci TSRMLS_CC, 2, &ob_data, &ob_mode);
 			
-#define PHP_OUTPUT_USER_SUCCESS(retval) (retval && (Z_TYPE_P(retval) != IS_NULL) && (Z_TYPE_P(retval) != IS_BOOL || Z_BVAL_P(retval)))
+#define PHP_OUTPUT_USER_SUCCESS(retval) (retval && !(Z_TYPE_P(retval) == IS_BOOL && Z_BVAL_P(retval)==0))
 			if (SUCCESS == zend_fcall_info_call(&handler->func.user->fci, &handler->func.user->fcc, &retval, NULL TSRMLS_CC) && PHP_OUTPUT_USER_SUCCESS(retval)) {
 				/* user handler may have returned TRUE */
 				status = PHP_OUTPUT_HANDLER_NO_DATA;



If HEAD's current behaviour is as intended, the docs should be updated to describe the new behaviour in PHP6.

Reproduce code:
---------------
<?php
function return_null($string) {
	return null;
}

ob_start('return_null');
echo "You shouldn't see this.\n";
ob_end_flush();

echo 'done';
?>


Expected result:
----------------
done

Actual result:
--------------
You shouldn't see this.
done