i have tried another code by store whole url in variable.It's give me error like "Uncaught SyntaxError: Unexpected end of input"
<?php
$url=$item['item_url'];
$title=$item["item_title"];
$str_url='http://www.facebook.com/sharer.php?u='.$url.'&t='.$title;
?>
<script>
var str_url = "'<?php echo $str_url; ?>'";
var title = "'<?php echo $title; ?>'";
var social_icon ="<div class='display-social-button'><a class='uk-icon-button uk-icon-facebook' onclick='popupCenter('"+ str_url + "','" + title + "','450','450');' href='javascript:void(0);'></a></div>";
</script>