1.php:
<?php
  print "hi";
?>

.htaccess:
ErrorDocument 406 /1.php

HTTP Request and Response:
D:\>netc bw 80
HEAD /index HTTP/1.1
Host: bw
Accept-Language: en

HTTP/1.1 200 OK
Date: Tue, 01 Aug 2000 01:36:34 GMT
Server: Apache/1.3.12 (Win32)
Alternates: {"index.html" 1 {type text/html} {charset iso-8859-1} {language de} {length 4698}}
Vary: negotiate
TCN: list
X-Powered-By: PHP/4.0.2-dev
Content-Type: text/html
Content-Language: de

hi
[...]

RFC 2616 forbids response bodys for HEAD requests.

mod_php 4.0.2-dev (build last week)