PHP: If internet explorer 6, 7, 8 , or 9

This is what I ended up using a variation of, which checks for IE8 and below:

if (preg_match('/MSIE\s(?P<v>\d+)/i', @$_SERVER['HTTP_USER_AGENT'], $B) && $B['v'] <= 8) {
    // Browsers IE 8 and below
} else {
    // All other browsers
}

A version that will continue to work with both IE10 and IE11:

preg_match('/MSIE (.*?);/', $_SERVER['HTTP_USER_AGENT'], $matches);
if(count($matches)<2){
  preg_match('/Trident\/\d{1,2}.\d{1,2}; rv:([0-9]*)/', $_SERVER['HTTP_USER_AGENT'], $matches);
}

if (count($matches)>1){
  //Then we're using IE
  $version = $matches[1];

  switch(true){
    case ($version<=8):
      //IE 8 or under!
      break;

    case ($version==9 || $version==10):
      //IE9 & IE10!
      break;

    case ($version==11):
      //Version 11!
      break;

    default:
      //You get the idea
  }
}

You can check the HTTP_USER_AGENT server variable. The user agent transfered by IE contains MSIE

if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== false) { ... }

For specific versions you can extend your condition

if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 6.') !== false) { ... }

Also see this related question.

Here is a great resource for detecting browsers in php: http://php.net/manual/en/function.get-browser.php

Here is one of the examples that seems the simplest:

<?php
function get_user_browser()
{
    $u_agent = $_SERVER['HTTP_USER_AGENT'];
    $ub = '';
    if(preg_match('/MSIE/i',$u_agent))
    {
        $ub = "ie";
    }
    elseif(preg_match('/Firefox/i',$u_agent))
    {
        $ub = "firefox";
    }
    elseif(preg_match('/Safari/i',$u_agent))
    {
        $ub = "safari";
    }
    elseif(preg_match('/Chrome/i',$u_agent))
    {
        $ub = "chrome";
    }
    elseif(preg_match('/Flock/i',$u_agent))
    {
        $ub = "flock";
    }
    elseif(preg_match('/Opera/i',$u_agent))
    {
        $ub = "opera";
    }

    return $ub;
}
?>

Then later in your code you could say something like

$browser = get_user_browser();
if($browser == "ie"){
    //do stuff
}

I do this

$u = $_SERVER['HTTP_USER_AGENT'];

$isIE7  = (bool)preg_match('/msie 7./i', $u );
$isIE8  = (bool)preg_match('/msie 8./i', $u );
$isIE9  = (bool)preg_match('/msie 9./i', $u );
$isIE10 = (bool)preg_match('/msie 10./i', $u );

if ($isIE9) {
    //do ie9 stuff
}