<?php
function isMobile() {
return preg_match("/(android|avantgo|blackberry|bolt|boost|cricket|docomo|fone|hiptop|mini|mobi|palm|phone|pie|tablet|up\.browser|up\.link|webos|wos)/i", $_SERVER["HTTP_USER_AGENT"]);
}
if(isMobile()){?>
<a href="viber://add?number=xxxxxxxxxxx">Viber</a>
<?php
}else{?>
<a href="viber://chat?xxxxxxxxxxx">Viber</a>
<?php } ?>