There is a simple way to get request headers from Apache even on PHP running as a CGI. As far as I know, it's the only way to get the headers "If-Modified-Since" and "If-None-Match" when apache_request_headers() isn't available. You need mod_rewrite, which most web hosts seem to have enabled. Put this in an .htacess file in your web root:
RewriteEngine on
RewriteRule .* - [E=HTTP_IF_MODIFIED_SINCE:%{HTTP:If-Modified-Since}]
RewriteRule .* - [E=HTTP_IF_NONE_MATCH:%{HTTP:If-None-Match}]
The headers are then available in PHP as
<?php
$_SERVER['HTTP_IF_MODIFIED_SINCE'];
$_SERVER['HTTP_IF_NONE_MATCH'];
?>
I've tested this on PHP/5.1.6, on both Apache/2.2.3/Win32 and Apache/2.0.54/Unix, and it works perfectly.
Note: if you use RewriteRules already for clean URLs, you need to put the above rules AFTER your existing ones.
