$proxy = new SoapClient('http://magentohost/api/soap/?wsdl');
$sessionId = $proxy->login('apiUser', 'apiKey');
$productId = 2;
$customerId = 10002;
$store = 'default';

// Add tags
$data = array('product_id' => $productId, 'store' => $store, 'customer_id' => $customerId, 'tag' => "First 'Second tag' Third");
echo "Adding Tag... ";
$addResult = $proxy->call(
    $sessionId,
    "product_tag.add",
    array($data)
);
echo ((count($addResult) == 3) ? "Done!" : "Fail!");
echo "<br />";
print_r($addResult);
$tagId = reset($addResult);

// Get tag info
echo "<br />Get Tag Info (id = $tagId)... ";
$infoResult = $proxy->call(
    $sessionId,
    "product_tag.info",
    array($tagId, $store)
);
echo "Done!<br />";
print_r($infoResult);

// Update tag data
$data = array('status' => -1, 'base_popularity' => 12, 'name' => 'Changed name');
echo "<br />Update Tag (id = $tagId)... ";
$updateResult = $proxy->call(
    $sessionId,
    "product_tag.update",
    array($tagId, $data, $store)
);
echo ($updateResult ? "Done!" : "Fail!");

// Retrieve list of tags by product
echo "<br />Tag list for product with id = $productId... ";
$listResult = $proxy->call(
    $sessionId,
    "product_tag.list",
    array($productId, $store)
);
echo (count($listResult) ? "Done!" : "Fail!");
echo "<br />";
print_r($listResult);

// Remove existing tag
echo "<br />Remove Tag (id = $tagId)... ";
$removeResult = $proxy->call(
    $sessionId,
    "product_tag.remove",
    array($tagId)
);
echo ($removeResult ? "Done!" : "Fail!");