if/else conditions in CSS?

here is list of operators are available

lt = less than operator lte = less than or equal to gt = greater than gte = greater than or equal to ! = the NOT operator () = subexpression operator & = the AND operator | = the OR operator true = evaluates to true false = evaluates to false

you can use this like this

<!--[if lt IE 7 ]>
  <p>Only less than IE 7 will see this</p>
<![endif]-->


<!--[if gte IE 6 ]>
 <p>Only  IE 6 and greater will see this</p>
<![endif]--> 

Check Session is Set or Not in Codeigniter ?

As per CodeIgniter Documentation If Session in not set it return FALSE .

if ($this->session->userdata('variable') !== FALSE) { 

         echo 'Variable is set';

 } else { 

        echo 'Variable is not set'; 

}

Find out whether an extension is loaded - PHP

Finds out whether the extension is loaded.

bool extension_loaded ( string $name )

Returns TRUE if the extension identified by name is loaded, FALSE otherwise.