[php] Detect if PHP session exists

The original code is from Sabry Suleiman.

Made it a bit prettier:

function is_session_started() {

    if ( php_sapi_name() === 'cli' )
        return false;

    return version_compare( phpversion(), '5.4.0', '>=' )
        ? session_status() === PHP_SESSION_ACTIVE
        : session_id() !== '';
}

First condition checks the Server API in use. If Command Line Interface is used, the function returns false.

Then we return the boolean result depending on the PHP version in use.

In ancient history you simply needed to check session_id(). If it's an empty string, then session is not started. Otherwise it is.

Since 5.4 to at least the current 8.0 the norm is to check session_status(). If it's not PHP_SESSION_ACTIVE, then either the session isn't started yet (PHP_SESSION_NONE) or sessions are not available altogether (PHP_SESSION_DISABLED).