Please add in function PHP_FUNCTION(imap_fetch_overview) in file php4/ext/imap/php_imap.c :

if (env->in_reply_to) {
         add_property_string(myoverview, "in_reply_to", env->in_reply_to, 1);
}

It can be tested with this:

$imap = imap_open('{servername:port}', 'login','passwd');
$header = imap_fetch_overview($imap,"1:20",0);
for($i=0;$i<count($header);$i++) {
  print $header[$i]->in_reply_to."\n";
}

Thanks.