Check if PHP Session has already started

Recommended way for versions of PHP >= 5.4.0

<?PHP
    if (session_status() == PHP_SESSION_NONE) {
        session_start();
    }
?>

Source: http://www.php.net/manual/en/function.session-status.php

For versions of PHP < 5.4.0

<?PHP
    if(session_id() == '') {
        session_start();
    }
?>