How do I dynamically insert a CSS class using PHP?

Question

What's wrong with the following code? I would like to dynamically insert "current" CSS class when the respective <li> element is clicked. Thanks!

<?php 
$pg = $_SERVER['PHP_SELF']; 
?>

<section>
    <aside>       
        <nav>
            <ul>
                <li class="<?php if($pg == 'index.php?page=about.php'){echo 'current';} ?>"><a href="index.php?page=about.php">About</a></li>                       
            <li class=""><a href="">Services</a></li> 
            </ul>
        </nav>
    </aside>
<section>

Answer 1

Try with:

<?php 
$pg = $_GET['page'];

$allow = array('about.php', 'main.php', 'other.php');
if ( !in_array($pg, $allow) ) {
    $pg = 'default_value';
}
?>

<section>
    <aside>       
        <nav>
            <ul>
                <li class="<?php if($pg == 'about.php'){echo 'current';} ?>"><a href="index.php?page=about.php">About</a></li>                       
            <li class=""><a href="">Services</a></li> 
            </ul>
        </nav>
    </aside>
<section>

Answer 2

<li class="<?php echo ($_GET['page'] == 'about.php')? 'current' : 'normal'; ?>">
  <a href="index.php?page=about.php">About</a>
</li>

Answer 3

$_SERVER['PHP_SELF'] doesn't return the $_GET vars (?page=about.php)

Answer 4

$_SERVER['PHP_SELF'] will start with / and will not include the query string.

You should probably check $_GET['page'] instead.