tests/lang/bug21094.phpt also failed on W2k server:

---- EXPECTED OUTPUT
[1024] errstr: test, errfile: %s, errline: %d
---- ACTUAL OUTPUT
Notice: test in D:\work\php5\tests\lang\bug21094.php on line 10
---- FAILED

This can be fixed quite easily:

Index: Zend/zend_builtin_functions.c
===================================================================
RCS file: /repository/ZendEngine2/zend_builtin_functions.c,v
retrieving revision 1.184
diff -u -r1.184 zend_builtin_functions.c
--- Zend/zend_builtin_functions.c       8 Apr 2003 18:22:32 -0000       1.184
+++ Zend/zend_builtin_functions.c       18 Apr 2003 21:49:51 -0000
@@ -967,13 +967,21 @@
 ZEND_FUNCTION(set_error_handler)
 {
        zval **error_handler;
+       char *error_handler_name;
        zend_bool had_orig_error_handler=0;
 
        if (ZEND_NUM_ARGS()!=1 || zend_get_parameters_ex(1, &error_handler)==FAILURE) {
                ZEND_WRONG_PARAM_COUNT();
        }
 
-       convert_to_string_ex(error_handler);
+       if (!zend_is_callable(*error_handler, 0, &error_handler_name)) {
+               zend_error(E_WARNING, "%s() expects argument 1, '%s', to be a valid callback",
+                                  get_active_function_name(TSRMLS_C), error_handler_name);
+               efree(error_handler_name);
+               return;
+       }
+       efree(error_handler_name);
+
        if (EG(user_error_handler)) {
                had_orig_error_handler = 1;
                *return_value = *EG(user_error_handler);

This bug has been fixed in CVS.

In case this was a PHP problem, snapshots of the sources are packaged
every three hours; this change will be in the next snapshot. You can
grab the snapshot at http://snaps.php.net/.
 
In case this was a documentation problem, the fix will show up soon at
http://www.php.net/manual/.

In case this was a PHP.net website problem, the change will show
up on the PHP.net site and on the mirror sites in short time.
 
Thank you for the report, and for helping us make PHP better.