toadhall - 9:40 pm on Apr 19, 2002 (gmt 0)

Even when spoofed "Opera" will show up in the UA...

Mozilla/3.0 (Windows 98; U) Opera 6.0 [en]
Mozilla/4.76 (Windows 98; U) Opera 6.0 [en]
Mozilla/5.0 (Windows 98; U) Opera 6.0 [en]
Opera/6.0 (Windows 98; U) [en]

...so David's solution looks good, or here's another:

$UA = getenv(HTTP_USER_AGENT);
if (strstr($UA,"Opera")) {
echo ("You're using Opera.");
}