jQuery : getting PHP variables by AJAX

you missed " operator there

var fullLink = "<?php echo $_SESSION['fullLink']; ?>";
  //-----------^------------------------------------^ here

this will print the fulllink as string and var fullLink will get that value as string...

Providing this is within a PHP file you'll just need to add the " operator.

var fullLink = "<?php echo $_SESSION['fullLink']; ?>";

That will work providing that $_SESSION['fullLink'] exists.

UPDATE

There must be some other issue in your code, I have just created a test script using the fullLink from your source example, and the window.open() code you provided. It worked fine with just these parts:

<script>
    var fullLink = "http://www.facebook.com/sharer.php?s=100&p[url]=localhost:8080/BetaFolioBlogOOP??/Blog/post/5-2e-post&p[title]=2e post&p[summary]= Voici mon deuxi&egrave;me post question de voir si le tout marche bien ! ";
    window.open(fullLink, "Facebook_share", "menubar=1,resizable=1,width=600,height=500");      
</script>

Try

var fullLink = <?php echo json_encode($_SESSION['fullLink']); ?>;

PHP : calling javascript function

func($_SESSION['fullLink']);

or better:

if (isset($_SESSION['fullLink']))
{ func($_SESSION['fullLink']); }

Javascript:

function func(link){

var fulllink = link;
...}