I can't save session as array

I can't save session $_SESSION["kokiai"] as array.
And I can't use $_SESSION["kokiai"] on other page, what is the problem? What I need to add? Thanks for the answers.
Btw, here is a code:

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
Pasirinkite prekes:<br />
<select name="products[]" multiple="multiple" size="3">
<option value="ferrari">ferrari</option>
<option>audi</option>
</select><p>
<input type="submit" value="Claim it" />
</form>
</html>
</body>
<?php
session_start();
if (isset($_POST["products"])){
$_SESSION["kokiai"] = serialize($_POST["products"]);
echo "<br>Your products was registered";
} else {
echo "<br>You haven't chosen any products yet.";
}
?><br>
<A href="la.php"> See what you chose</A>

PHP:

 

session_start() must go at the top. Before sending ANYTHING (except other headers) to the user's browser. No white spaces, no HTML... nothing.

And don't use $_SERVER['PHP_SELF'] like that. It allows XSS attacks.