As @HendrikBrummermann already pointed out in the comments, it was an attempt to exploit register_globals on a PHP enabled web server.
The server making the request that ended up in your web server's logs, so it turned out, was merely an infected web server running in zombie mode, running a stealth bot checking for possible attack vectors on unsuspecting web servers throughout Internet on behalf of larger botnet, controlled through a LeaseWeb B.V., Netherlands hosted command and control server with an IP of 188.8.131.52 hosting hundreds of suspicious domains. According to Spamhaus CBL, the server with an IP 184.108.40.206 located in Budapest, Hungary was infected with Hermes trojan:
Hermes is a banking trojan aimed to steal credentials for online
banking accounts. It spreads through hijacked websites (drive-by
exploits) and malicious email attachments.
I've been analyzing this botnet's command and control IP a bit, and it is running a VNC Remote Desktop on port 5900, meaning it's also controlled from a remote location that is unknown to us. But that's beyond the point, as it's quite normal for such criminal networks to simply switch to another hosting company, if and when the one they rely on now is required by law to take action. As hosting companies don't particularly like to take action until they absolutely have to, this might not be on the top of their to-do list. What will more likely happen is that the botnet's current IP will be marked as blacklisted in too many RBLs to feel comfortable operating from (live status), and move its command and control to another hosting company.
So, in short, the suspicious line in your web server's logs was just one of the attack vectors that such botnets would be scanning for, looking for ways to exploit web servers. If the takeover was successful, the next person that opened your web page would've been redirected to the exploited zombie server in Budapest, Hungary and the botnet would've known your web server was thus exploitable by checking the referral string. However, your web server was obviously not the low hanging fruit they were hoping to exploit this easily, at least not through this particular PHP security hole. Botnet cares not and has moved on to scanning thousands of other hosts by the time you finished reading my last sentence.
Just another day on the WWW. ;)