PHP Session handling Problem

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PHP

I am creating simple website. With the help of w3wchools website i developed some session handling scripts . that scripts are working well on my localhost. But its not working in uploaded server sometimes. Yesterday its worked in server. But its not working. after some time its working..

File 1

<?php
session_start();
$_SESSION["username"]="suyambu";
?>

file 2

<?php
session_start();
echo $_SESSION["username"];
?>

after i visit the file 1 i visited 2nd page..but no output.. :(

Please help me