<?php
    $filtered = array_filter($categories, function($cat) {
        return $cat['name'];
    });
    foreach (array_slice($filtered, 0, 5) as $cat) : ?>
        <li class="category">
            <a id="cat_<?php echo $cat['name']?>" href="?tag=<?php echo $cat['name']?>"><?php echo $cat['name']?></a>
        </li>

<?php endforeach ?>

$(document).ready(function(){
    $('#cat_<?php $cat[0]['name'];?>').find('a').trigger('click');
});

<ul class="navigation">
    <li class="category">
        <a id="cat_cars" href="?tag=cars">Cars</a>
    </li>
    <li class="category">
        <a id="cat_people" href="?tag=people">People</a>
    </li>
    <li class="category">
        <a id="cat_animal" href="?tag=animal">Animal</a>
    </li>
    <li class="category">
        <a id="cat_web" href="?tag=web">Web</a>
    </li>
    <li class="category">
        <a id="cat_forum" href="?tag=forum">Forum</a>
    </li>
</ul>

$(document).ready(function(){
    $('#cat_').find('a').trigger('click');
});