Php dentro de javascript Shared facebook 

 $url="www.example.com";
<script>
      function share_tanck(){
        FB.ui({
          method: 'feed',
          link: '<?php echo $url; ?>',
          caption: 'An example caption',
        }, function(response){});
      }
    </script>
    <a onclick="share_tanck()">Click me</a> ------------------..............------------------