a.php
<?php
session_start();
$_SESSION["username"] = $_POST["username"];
?>
<a href="b.php">跳转到b.php</a>
b.php
<?php
session_start();
echo $_SESSION["username"];
?>
<?php
session_start();
$_SESSION["username"] = $_POST["username"];
?>
<a href="b.php">跳转到b.php</a>
b.php
<?php
session_start();
echo $_SESSION["username"];
?>
session_start();
$username = "abc";
session_register("username");
?>
<a href="b.php">b.php</a>b.php
<?
session_start();
echo $_SESSION["username"]; //注意SESSION一定要大写!
?>