rating_header.phtml

<?php
/**
* Hint: I stored the rating_header.phtml under /template/rating.
* Load the template whereever you want by doing <?php echo $this->getLayout()->createBlock('core/template')->setTemplate('rating/rating_header.phtml')->toHtml(); ?> in your template file.
*/

/**
* Getting reviews collection object.
*/

$reviews = Mage::getModel('review/review')
->getResourceCollection()
->addStoreFilter(Mage::app()->getStore()->getId()) 
->addStatusFilter(Mage_Review_Model_Review::STATUS_APPROVED)
->setDateOrder()
->addRateVotes();

/**
* Getting average of ratings/reviews.
*/

$avg = 0;
$totalrv = 0;
$totalrvper =0;

/**
* Check if the current page is a product page.
*/

$isProductPage = false;

if ( Mage::registry('current_product') )
{
    $isProductPage = true;
}


if (count($reviews) > 0) 
{
    foreach ($reviews->getItems() as $review) 
    {
        foreach( $review->getRatingVotes() as $vote ) 
        {

            $totalrv = $totalrv+$vote->getValue();
            $totalrvper = $totalrvper + $vote->getPercent(); 

        }
    }

    // Total reviews.
    $totalrv = $totalrv / count($reviews->getItems());

    // Total reviews in percent.
    $totalrvper = ($totalrvper)/ count($reviews->getItems() );

    // Total reviews rounded.
    $avgrate = round($totalrv, 1);
}
?>


<div class="header-rating">
    <div class="ratings">
        <div class="rating-box">
            <div class="rating" style="width:<?php echo $totalrvper; ?>%"></div>
        </div>
    </div>
    <?php if ( $isProductPage ): ?>
        <div class="rating-text">
            <span>Unsere Kunden bewerten uns mit <?= $avgrate ?> von 5 Sternen, basierend auf <a title="Alle Produktbewertungen durch unsere Kunden" href="/produktbewertungen-durch-unsere-kunden"><?= count($reviews->getItems()) ?> Produktbewertungen</a>.</span>
        </div>
    <?php else: ?>
        <div class="rating-text" itemprop="aggregateRating" itemscope="" itemtype="http://schema.org/AggregateRating">
            <meta itemprop="itemReviewed" content="Your Company">
            <meta itemprop="ratingValue" content="<?= $avgrate ?>">
            <meta itemprop="reviewCount" content="<?= count($reviews->getItems()) ?>">
            <span>Unsere Kunden bewerten uns mit <?= $avgrate ?> von 5 Sternen, basierend auf <a title="Alle Produktbewertungen durch unsere Kunden" href="/produktbewertungen"><?= count($reviews->getItems()) ?> Produktbewertungen</a>.</span>
        </div>
    <?php endif; ?>
</div>