How to using replace with variable?

Question

$aaa = preg_replace('/<%var.\$(fullname).%>/is', $_SESSION["\1"], $aaa);

Answer 1

You can use preg_replace_callback instead:

$line = preg_replace_callback(
    '/<%var.\$(fullname).%>/is',
    create_function(
        '$matches',
        'return $_SESSION[$matches[1]];'
    ),
    $line
);

Though I would be wary of where you're getting the input string from: I would be wary if it's coming from users (it looks like it might be coming from some kind of configuration or template file, though, which is probably OK).