PHP if语句不显示HTML

So I'm checking if a user is logged in, and if so I'll remove the signup forms. So I thought I'd do it with if else statements. So here's my code

<?php if($_SESSION['loggedin'] === false): ?>
<h1>Sign Up</h1>
<?php elseif($_SESSION['loggedin'] === true):?>
<h1>Welcome</h1>
<?php endif;?>

For some reason all I get is a blank page. I'm using

error_reporting(E_ALL);
ini_set('display_errors', '1');

To display errors, but I'm not getting anything. On another page when doing a var_dump on $_SESSION['loggedin' I get bool(true), so I know that I'm logged in and expect Welcome. I feel like its a syntax error. Any ideas?

If you tail your server logs (e.g. tail -F logs/php_error.log) you'll probably see something like:

PHP Notice: Undefined variable: _SESSION

Try changing the code to:

if (isset( $_SESSION['loggedin'] ) && $_SESSION['loggedin'] == true )
{
  echo 'Welcome';
} else {
  echo 'Sign up';
}

Personally I'd also recommend changing your sign out logic to use unset($_SESSION) that way you only have to check for:

if (isset( $_SESSION['loggedin'] ) ){
  echo 'Welcome';
} else {
  echo 'Sign up';
}

Try this way:

<?php if(!$_SESSION['loggedin']) { ?>
<h1>Sign Up</h1>
<?php } elseif($_SESSION['loggedin']) { ?>
<h1>Welcome</h1>
<?php } ?>

Try this:

<?php if(isset($_SESSION['loggedin']) && $_SESSION['loggedin'] === true): ?>
<h1>Welcome</h1>
<?php else:?>
<h1>Sign Up</h1>
<?php endif;?>

It's possible that neither code path gets executed if $_SESSION['loggedin'] isn't a boolean because the === comparison checks type and value. (See documentation for details)

A more safe and sane approach would be:

<?php if($_SESSION['loggedin'] === true): ?>
<h1>Welcome</h1>
<?php else:?>
<h1>Sign Up</h1>
<?php endif;?>

Try adding an isset() to your if test.

<?php if(!isset($_SESSION['loggedin']) || $_SESSION['loggedin'] === false): ?>
<h1>Sign Up</h1>
<?php else: ?>
<h1>Welcome</h1>
<?php endif;?>

it is not resulting due to Undefined variable: _SESSION error. so you can utilize any ignorant (@ or &) Ex. @$_SESSION['loggedin'] OR &$_SESSION['loggedin']