TBH your whole code looks like a lot of security risks. I thought about making a proper list of vulnerabilities, but this would require a lot of time from me (since your site does look complex) and the bounties look a bit "uncertain". So I am not sure if I want to put like a few days of time into it TBH.
Just in general you REALLY should adjust this:
* NEVER ever use the "quote()" shit for protection of SQL injection. Use prepared statements (lookup PDO->prepare()) instead.
* Use CSRF protection probably just ALWAYS whenever there is user input. I am pretty sure I can steal the escrow coins by sending the escrow an URL that loads an iframe/post-form/etc to send the coins to me. This requires user interaction (escrow has to open an URL), but can be pretty easy/doable.
* IMO you shouldn't strip_tags before putting it in the DB. Just always sanitize the output to the user. A template system can help with this, some will HTML encode every variable by default.
* Best to make most scripts NOT public for visitors to access. Basically you should only have .htaccess + bootstrapper (index.php) in your public_html together with CSS/JS/images. All PHP should be outside of it and just loaded by index. In your situation, you could at least hide the cron/classes/includes/libs/scripts. For example I can get your RPC password here:
http://empirecoin.org/scripts/getinfo.php That is probably some test file which you forgot to delete (and I assume the bitcoind is not running at the moment), but could be serious problem.
* Just in general your code would be much easier to read (= easier to see bugs) if it's divided into MVC structure.
* Functions like rand() and mt_rand() are not cryptographically secure. It is possible to hack accounts on your site by using the "reset password" function and cracking mt_rand. Search for random_bytes() - I believe that is cryptographically secure in PHP lately.
All of those things, are generally taken care of by a PHP framework. That is why using a PHP framework is pretty great. I really like Laravel lately. But converting your site to a PHP framework might take serious time.
On the positive side: it seems to have a lot of features and I can always appreciate open-source work.
Another SQL Injection is detected in "get_session.php" line 15.
$session_key is not escaped.
I am pretty sure session_id() verifies any session ID from cookie etc? I don't think you can get values with " with that. OP should still use prepared statements though.