GET parameters interfere with my plugin settings

Why does the Settings API send those parameters when submitting the settings form?

Quite frankly, it doesn’t. As you already mentioned, your form simply POST’s to options.php, which in turn handles the request, updates the database, and then redirects back to the referer.

How the referer is fetched is down to the function wp_get_referer();

function wp_get_referer() {
    $ref="";
    if ( ! empty( $_REQUEST['_wp_http_referer'] ) )
        $ref = $_REQUEST['_wp_http_referer'];
    else if ( ! empty( $_SERVER['HTTP_REFERER'] ) )
        $ref = $_SERVER['HTTP_REFERER'];

    if ( $ref !== $_SERVER['REQUEST_URI'] )
        return $ref;
    return false;
}

That’s why you’re getting sent back to your options page with the action parameters still there – because they were present in $_SERVER['HTTP_REFERER'].

But you’ll also see that you can override this behaviour, by placing a hidden input inside your form like so;

<input type="hidden" name="_wp_http_referer" value="<?php echo admin_url( 'options-general.php?page=wordpress-file-monitor-plus' ); ?>" />

Now options.php will always redirect back to…

http://example.com/wp-admin/options-general.php?page=wordpress-file-monitor-plus

…regardless of whatever was in the query string previously.

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