why would setting redirect in if and else condition wont work in php?

Am baffled by a simple code even as experienced developer. Am trying to redirect a user depending on two condition. I have a variable called $isMobile which is boolean. Now below code cannot work if i add redirect but removing redirect header, it works

This does not work

if($isMobile){
    header('Location:http://localhost/myproject/mobile/login/');
    exit();
}
else{
    header('Location:http://localhost/myproject/pc/login/');
    exit();
}
//Error: too many redirects as if both conditions are true. Weird right?

This works

if($isMobile){
    echo 'redirect to mobile folder';
}
else{
    echo 'redirect to desktop folder';
}

What makes everything stop after adding both header redirects? I need to redirect users depending on their devices. Am so stuck here guys.