How to start session and what will happen if I start session twice?

How to start session in PHP

session_start();

?>

What will happen if I write session_start(); twice

session_start();
echo session_id();

echo "
";

session_start();
echo session_id();

?>

Reason : As per the above code writing it twice will not make any difference to the code and it will continue to keep the previous generated session_id();