PHP Deprecated: Non-static method should not be called statically

My debug.log has the following message that points to /wp-includes/shortcodes.php on line 319, which is part of WordPress core and probably isn’t where the real error is in the code.

PHP Deprecated:  Non-static method RSSjbClass::RSSjb_funct() should not be called statically in /.../wp-includes/shortcodes.php on line 319

I’ve looked at line 319 and it doesn’t help me find or solve the problem. I searched the code and found this with the Class and funct but I don’t know what is non-static being called statically.

/**
 * Add shortcode to wordpress
 */
// add_shortcode('RSSjb', 'RSSjb_funct');
add_shortcode('RSSjb', array('RSSjbClass', 'RSSjb_funct'));
/**
 * Shortcode Class and Function
 */
class RSSjbClass {
    function RSSjb_funct($atts) {
        extract(shortcode_atts(array(
        // attributes for Google News
        "gsearch" => '',
        "topic" => '',
        "location" => '',
        "geo" => '',
        "feed" => '', // required
        "filter" => '',
        "num" => '5',
        "ltime" => '',
        "list" => 'ul',
        "target" => '_blank',
        "pubdate" => 'false',
        "pubtime" => 'false',
        "dformat" => get_option('date_format'),
        "tformat" => get_option('time_format'),
        "pubauthor" => 'true',
        "excerpt" => 'false',
        "charex" => '',
        "chartle" => '',
        "title" => '',
        "link" => 'false',
        "sort" => 'false',
        "cachefeed" => '3600'
    ), $atts));

followed by several “ifs” and then

    // call the function to read and display the feed items list
        return $tle . rssjb_List($feed, $filter, $num, $ltime, $list, $target, $pubdate, $pubtime, $dformat, $tformat, $pubauthor, $excerpt, $charex, $chartle, $sort, $cachefeed);
    } else {
        return '<br>RSS or Atom Feed URL not provided. This shortcode does require the attribute feed or location (if Google News).<br /> Ex: <code>[RSSjb feed = "http://your-rss-or-atom-feed-URL-here"]</code>.'; 
    }
    }
}

How do I modify that to fit with what is in /wp-includes/shortcodes.php line 319, which is:

    $output = $m[1] . call_user_func( $shortcode_tags[ $tag ], $attr, $content, $tag ) . $m[6];

    /**
     * Filters the output created by a shortcode callback.
     *
     * @since 4.7.0
     *
     * @param string       $output Shortcode output.
     * @param string       $tag    Shortcode name.
     * @param array|string $attr   Shortcode attributes array or empty string.
     * @param array        $m      Regular expression match array.
     */
    return apply_filters( 'do_shortcode_tag', $output, $tag, $attr, $m );
}

I searched the PHP Manual and it doesn’t have anything about shortcodes.

Shortcodes are a WordPress thing, hence not being in the PHP manual. What you do want to look at in the PHP manual is the section on callbacks. add_shortcode() accepts a shortcode name, and a callback. In that section of the PHP manual you can see the 3 main types of callbacks:

// Type 1: Simple callback
call_user_func('my_callback_function');

// Type 2: Static class method call
call_user_func(array('MyClass', 'myCallbackMethod'));

// Type 3: Object method call
$obj = new MyClass();
call_user_func(array($obj, 'myCallbackMethod'));

Your callback function for your shortcode is the 2nd type:

add_shortcode('RSSjb', array('RSSjbClass', 'RSSjb_funct'));

This is the same as calling RSSjbClass::RSSjb_funct, which is how you call a static method, but in your class the RSSjb_funct() method isn’t defined statically:

function RSSjb_funct($atts) {

So the quickest fix is to define that method as static:

public static function RSSjb_funct($atts) {

The other option would be to add the shortcode within the class using $this, or to create an instance of the class and then use that in add_shortcode() like Type 3 above.