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<?php
// Note: To use cookie-based sessions, session_start()
// must be called before outputting anything to the browser.
@session_start();
?>
<!doctype html>
<body>
<?php
$REQUEST=$_REQUEST;//['action'=>'login','user'=>'user@gmail.com','password'=>'secret'];//$_REQUEST;
$passwords=['user@gmail.com'=>'secret'];

// actions (login, logout)
if(@$REQUEST['action']=='login' &&
@$passwords[@$REQUEST['user']]==@$REQUEST['password'])
$_SESSION['user']=$REQUEST['user'];
elseif(@$REQUEST['action']=='logout')
$_SESSION['user']='';

if(@$_SESSION['user']!=''){
?>
<!-- logged in -->
<form method="post">
<input type="hidden" name="action" value="logout">
<input type="submit" value="logout <?=$_SESSION['user']?>">
</form>
protected content <br>
... <br>... <br>... <br>... <br>... <br>... <br>... <br>... <br>
<?php
}else{
?>
<!-- logged out -->
<form method="post">
<input type="hidden" name="action" value="login">
<input type="email" name="user" placeholder="username">
<input type="password" name="password" placeholder="password">
<input type="submit" value="login">
</form>
<?php
}
?>
</body>