[NTLDR]

PHP Code:

if ($_SERVER['SERVER_PORT'] == 80) {
    $secure = 'http';
} elseif ($_SERVER['SERVER_PORT'] == 443) {
    $secure = 'https';
} 


Should do the trick. However there are probably very few, if any references to http://..... in the source and templates so a better option IMO would be to just change the value of the boards URL:

PHP Code:

if ($_SERVER['SERVER_PORT'] == 443) {
    $vboptions['bburl'] = 'https://mydomain.com/forums';
} 


Of course you'll still get errors for inline images in [IMG] code if there are on HTTP but thats unavoidable unless you disable it.