php

PHP >= 5.4.0 , PHP 7

if (session_status() == PHP_SESSION_NONE) {
    session_start();
}

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

For versions of PHP < 5.4.0

if(session_id() == '') {
    session_start();
}
PHP Check if session has started or not, then session_start()