<?php
$nav = <<<NAV
    <nav id="nav">
        <ul>
            <li><a href="/">Home</a></li>
            <li><a href="/section1/">Section 1</a></li>
            <li><a href="/section2/">Section 2</a></li>
        </ul>
    </nav>
NAV;

$lines = explode("n", $nav);
foreach ($lines as $line) {
    $current = false;
    preg_match('/href="([^"]+)"/', $line, $url);
    if (substr($_SERVER["REQUEST_URI"], 0, 5) == substr(@$url[1], 0, 5)) {
        $line = str_replace('<a h', '<a  class="active" h', $line);
    }
    echo $line."n";
}
?>

<?php
$nav = <<<NAV
    <nav id="nav">
        <ul>
            <li><a href="/">Home</a></li>
            <li><a href="/section1/">Section 1</a>
                <ul>
                    <li><a href="/section1/child-page-1">Child Page 1</a></li>
                    <li><a href="/section1/child-page-2">Child Page 2</a></li>
                </ul>
            </li>
            <li><a href="/section2/">Section 2</a></li>
        </ul>
    </nav>
NAV;

$lines = explode("n", $nav);
foreach ($lines as $line) {
    $current = false;
    preg_match('/href="([^"]+)"/', $line, $url);
    if (substr($_SERVER["REQUEST_URI"], 0, 5) == substr(@$url[1], 0, 5)) {
        $line = str_replace('<a h', '<a  class="active" h', $line);
    }
    echo $line."n";
}
?>