What is the best way to detect IE browsers 8 and below?

I would suggest using that plugin, but slightly modifying it for your needs. For example, you can find the code block in php-browser-detection.php that does the IE7 check:

function is_IE7 (){
    $browserInfo = php_browser_info();
    if(isset($browserInfo['browser']) && $browserInfo['browser']=='IE' && $browserInfo['majorver'] == '7')
        return true;
    return false;   
}

To create a version that will detect IE 8, add a new function that does the same thing, but just with the majorver set to 8:

function is_IE8 (){
    $browserInfo = php_browser_info();
    if(isset($browserInfo['browser']) && $browserInfo['browser']=='IE' && $browserInfo['majorver'] == '8')
        return true;
    return false;   
}