How secure is a header redirect? Can it be bypassed?

Question

How secure are header redirects? As I understand it, the header information instructs the browser what to do. Therefore, if the header location is set to some other place, can the browser still be made to land on the initial page?

if(!$loggedin){
header('Location: someotherpage.php')
}

If someone, (cough cough) forgot to use an exit command after the header, could someone still land on that page?

Answer 1

If someone, (cough cough) forgot to use an exit command after the header, could someone still land on that page?

Sure - rather than land on that page (which he already has), forgetting an exit() means the client can still see any data emitted after those lines. Nobody can force the client to leave immediately after receiving a Location: header. That action is entirely voluntary.