PHP - 会话不会导致带标头的重定向

I am trying to make the user get redirected to the index.html page in case the $_SESSION['loggedin'] is already set. For some reason, it doesn't redirect the user, although the $_SESSION['loggedin'] when echoed returns a value.

if(!empty($_SESSION['loggedin'])) { header('Location: index.html');};

What am I doing wrong here?

header('Location: index.html'); sets an http header which needs to be set before any content is sent, i.e. before any echo. Also you should terminate your script after you set the location header.