IE 6 detect with PHP

// IE6 string from user_agent
$ie6 = "MSIE 6.0";

// detect browser
$browser = $_SERVER['HTTP_USER_AGENT'];

// yank the version from the string
$browser = substr("$browser", 25, 8);

// if IE6 set the $alert
if($browser == $ie6){
echo 'yes IE 6';
}

Ref: http://stackoverflow.com/questions/1042176/php-getbrowser-how-to-identify-ie7-versus-ie6