<?php
if (ereg("MSIE ([0-9]+)\.([0-9]+)", $HTTP_USER_AGENT, $ver)) {
    $version = "$ver[1].$ver[2]";
    if ($version < 5) {
        echo "古いです:", $HTTP_USER_AGENT;
    } else {
        echo "OK", $HTTP_USER_AGENT;
    }
} else {
    echo "IE ではありません:", $HTTP_USER_AGENT;
}
?>