As the error says you need an instance of the class to use $this
. There are at least three possibilities:
Make everything static
class My_Plugin
{
private static $var="foo";
static function foo()
{
return self::$var; // never echo or print in a shortcode!
}
}
add_shortcode( 'baztag', array( 'My_Plugin', 'foo' ) );
But that’s not real OOP anymore, just namespacing.
Create a real object first
class My_Plugin
{
private $var="foo";
public function foo()
{
return $this->var; // never echo or print in a shortcode!
}
}
$My_Plugin = new My_Plugin;
add_shortcode( 'baztag', array( $My_Plugin, 'foo' ) );
This … works. But you run into some obscure problems if anyone wants to replace the shortcode.
So add a method to provide the class instance:
final class My_Plugin
{
private $var="foo";
public function __construct()
{
add_filter( 'get_my_plugin_instance', [ $this, 'get_instance' ] );
}
public function get_instance()
{
return $this; // return the object
}
public function foo()
{
return $this->var; // never echo or print in a shortcode!
}
}
add_shortcode( 'baztag', [ new My_Plugin, 'foo' ] );
Now, when someone wants to get the object instance, s/he just has to write:
$shortcode_handler = apply_filters( 'get_my_plugin_instance', NULL );
if ( is_a( $shortcode_handler, 'My_Plugin ' ) )
{
// do something with that instance.
}
Old solution: create the object in your class
class My_Plugin
{
private $var="foo";
protected static $instance = NULL;
public static function get_instance()
{
// create an object
NULL === self::$instance and self::$instance = new self;
return self::$instance; // return the object
}
public function foo()
{
return $this->var; // never echo or print in a shortcode!
}
}
add_shortcode( 'baztag', array( My_Plugin::get_instance(), 'foo' ) );