<?php
if(!empty($_POST)){
  //验证数据
  ……
  //插入数据库
  mysql_query(……);
  //将用户名存入会话
  session_start();
  $_SESSION['user'] = trim($_POST['username']);
}
?>
<form name="login" action="login.php" method="post">
  <input type="text" name="username" />
  <input type="password" name="password"  />
  <input type="submit" name="btn" value="login"  />
</form>

<?php
//其他页面调用登录用户
session_start();
echo "username-->>".$_SESSION['user'];
?>