Question

Can I set variables inside if-construct? In general, where is it allowed to set variables?

function set_login_session ( $passhash ) 
{
    $_SESSION['login']['logged_in'] = 1;
    if ( ERROR ) return false;
}

// Does it set the Session variable?
if ( set_login_session ( $passhash ) === false)
    return false;

Answer 1

Short answer

Yes

Long answer

If this script has called start_session() earlier (or the session.auto_start configuration flag is set) then session variable can be set anywhere by using superglobal $_SESSION array.