the basic structure or syntax of conditional statements is just..like plain HTML comments so therefore you can only use it inside an html file..

here are some examples of Internet Explorer Conditional Statement syntax

<!--[if IE 6]>
enter your instructions for ie6 here
<![endif]-->

<!--[if gte IE 5]>
enter your instructions for ie browers greater than or equal to ie5
<![endif]-->

<!--[if lte IE 6]>
enter your instructions for ie browsers lower or equal to ie6
<![endif]-->

<!--[if lt IE 6]>
enter your instructions for ie browsers lower than ie6
<![endif]-->

<!--[if gt IE 6]>
enter your instructions for ie6 here
<![endif]-->

this is very usefull for example if you want to separate a css file for different different versions
of internet explorer you can use this, or if you want to output messages to your site visitor telling
them that they are using ie. here are some examples

Using conditional statement to separate css stylesheet

<!--[if IE 6]>
<link rel="stylesheet" type="text/css" href="ie6.css">
<![endif]-->

<!--[if IE 7]>
<link rel="stylesheet" type="text/css" href="ie7.css">
<![endif]-->

Using Conditional statement to output text to the browser

<!--[if IE 6]>
Hi There! You are using Internet Explorer 6
<![endif]-->

<!--[if IE 5]>
Hi there! You are a legend your still using IE5?
<![endif]-->

Enjoy

here is the actual code that i use. .

<?php $browser = $_SERVER['HTTP_USER_AGENT']; ?>
<?php if (strstr($browser, "MSIE 6.0")) { ?>
<link rel="stylesheet" href="ie6style.css" type="text/css" media="screen" />
<?php }else{ ?>
<link rel="stylesheet" href="default.css" type="text/css" media="screen" />
<?php } ?>

$_SERVER['HTP_USER_AGENT'] will return the users browser information, which i inserted in the variable $browser, then i use the string function strstr which is String Find. for IE6 use “MSIE 6.0″, IE7 “MSIE 7.0″ for all versions of Internet Explorer just use “MSIE”.

to separate styles of other browser you may use the following in the string find function. .

IE – “MSIE”

Firefox – “Firefox”

Safari – “Safari”

Opera – “Opera”

Chrome – “Chrome”

for specific browser version just type space after the browser name then type the version number.

if you are separating style only for Internet Explorer you may also use Conditional Statement

this does not require PHP or Javacript.

the code is.

<!--[if IE]>
// put your ie stylesheet here
<![endif]-->

<!--[if !IE]-->
// put your default stylesheet here
<![endif]-->