用php fopen 编写了一个注册页面，当我要读取文件时应该怎么办？急，在线等，求救啊！！！

用php fopen 编写了一个注册页面，注册表保存在user.txt 文件里，当我要读取文件时应该怎么办？

这是我现在的注册页面：
<?php
if (count($_POST)<1) {
echo "<p>First add some POST key / value pairs to the end of this url</p>";
} else {
// Part 1: Process POST data
$newline = implode("|",$_POST)."\n";
// Part 2: Save POST data
$handle = fopen("users.txt", "a");
if (flock($handle,LOCK_EX))
{
fwrite($handle, $newline);
flock($handle,LOCK_UN);
fclose($handle);
echo "now, you can watch the <a href='product.php'>product page</a>.";
}
else
echo "<p>Someone else has locked users.txt. Giving up.</p>";
}
?>
求登录的代码，我写了一个，可是不对，应该是查询username 和 password，如果对上了就可以登录。：
<?php
session_start();
if(!isset($_SESSION['username']))
{
header("Location :login.php");
}
$username = $_POST['username'];
$password = $_POST['password'];
$file = fopen("users.txt","r");
$data=fgets($file);
fclose($file);
if($username == $data && $password == $data)
{
header("Location: product.html");
}else{
echo "<h1>log in failed!</h1>";
echo "<br/><br/>";
echo "<a href = 'login.php'>try again</a>";
}
?>

相关了解……