The second argument of the header() command is a boolean that indicates if a call to header() should replace a header of a similar name. Under IIS you get multiple headers regardless, see the examples below, note the number of Foo: headers in each:


Example Script (foo.php):
=========================

<?php
header('Foo: 123',true);
header('Foo: 456',true);
?>


Output from Apache/PHP:
=======================

tab@test% telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET /~tab/foo.php HTTP/1.0

HTTP/1.1 200 OK
Date: Thu, 01 Aug 2002 15:57:02 GMT
Server: Apache/1.3.26 (Unix) PHP/4.2.2 mod_ssl/2.8.9 OpenSSL/0.9.6a
X-Powered-By: PHP/4.2.2
Foo: 456
Connection: close
Content-Type: text/html


Output from IIS:
================
tab@test% telnet tab 80
Trying 100.100.30.164...
Connected to tab.bwlaw.co.uk.
Escape character is '^]'.
GET /foo.php HTTP/1.0

HTTP/1.1 200 OK
Server: Microsoft-IIS/5.0
Date: Thu, 01 Aug 2002 16:00:44 GMT
Content-type: text/html
X-Powered-By: PHP/4.2.2
Foo: 123
Foo: 456