+ * and PAGE_NAME in session and returns false. POST information is saved in
+ * 'session_expired_post' variable, PAGE_NAME is saved in 'session_expired_location'.
+ *
+ * This function optionally checks the referrer of this page request. If the
+ * administrator wants to impose a check that the referrer of this page request
+ * is another page on the same domain (otherwise, the page request is likely
+ * the result of a XSS or phishing attack), then they need to specify the
+ * acceptable referrer domain in a variable named $check_referrer in
+ * config/config.php (or the configuration tool) for which the value is
+ * usually the same as the $domain setting (for example:
+ * $check_referrer = 'example.com';
+ * However, in some cases (where proxy servers are in use, etc.), the
+ * acceptable referrer might be different. If $check_referrer is set to
+ * "###DOMAIN###", then the current value of $domain is used (useful in
+ * situations where $domain might change at runtime (when using the Login
+ * Manager plugin to host multiple domains with one SquirrelMail installation,
+ * for example)):
+ * $check_referrer = '###DOMAIN###';
+ * NOTE HOWEVER, that referrer checks are not foolproof - they can be spoofed
+ * by browsers, and some browsers intentionally don't send them, in which
+ * case SquirrelMail silently ignores referrer checks.