PHP menu with .active classes - php

Question

I'm trying to build a navigation in php and add an .active class to the <li> element if the page displays. I found this code and it works for the top level elements, i.e. it will apply the .active class to the <li> when the selected page loads.
<?php
$nav = <<<NAV
<nav id="nav">
<ul>
<li>Home</li>
<li>Section 1</li>
<li>Section 2</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";
}
?>
However, when I add some child pages to the html these child pages also inherit the .active class even when the parent page is displayed. Likewise, if a child page is displayed the parent also inherits the .active class.
<?php
$nav = <<<NAV
<nav id="nav">
<ul>
<li>Home</li>
<li>Section 1
<ul>
<li>Child Page 1</li>
<li>Child Page 2</li>
</ul>
</li>
<li>Section 2</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";
}
?>
Is there a way to adapt the php so only the <li> to the corresponding page inherits the .active class. Also if the .active element was a child then the parent <li> would get a class name like .active-trail?
Alternatively is there a better way to do this via another language like jQuery?
Really appreciate any help on this.