The called constructor method for WP_Widget is deprecated since version 4.3.0

I am also getting the same error And I fixed it in such a way

class Dokan_Category_Widget extends WP_Widget {

    /**
     * Constructor
     *
     * @return void
     **/
    public function __construct() {
        $widget_ops = array( 'classname' => 'dokan-category-menu', 'description' => __( 'Dokan product category menu', 'dokan' ) );
        $this->WP_Widget( 'dokan-category-menu', 'Dokan: Product Category', $widget_ops );
    }
}

As way of calling constructor in such way is deprecated in php 7, so I replaced calling way as $this->WP_Widget() with parent::__construct()

class Dokan_Category_Widget extends WP_Widget {

    /**
     * Constructor
     *
     * @return void
     **/
    public function __construct() {
        $widget_ops = array( 'classname' => 'dokan-category-menu', 'description' => __( 'Dokan product category menu', 'dokan' ) );
        //$this->WP_Widget( 'dokan-category-menu', 'Dokan: Product Category', $widget_ops );
        parent::__construct('dokan-category-menu', 'Dokan: Product Category', $widget_ops  );
    }
}

Since php 7 doesn't support php 4 object construct and was replaced with __construct() Wordpress developers created a notice message so the plugin developers would change the way their plugins work. As php 4 has been dead for a very long time there's no reason to use this style of object construct.

How to fix?

Option 1 - not going to upgrade to newer php versions

just add add_filter('deprecated_constructor_trigger_error', '__return_false');

to your functions.php file it will ignore those notices.

Option 2 - might upgrade to php 7 / prefer dealing with the issue rather than silencing it

If this is a third party plugin, beware that if you make the change yourself and the plugin developer releases an update then it will override your changes. Contacting the plugin developer to fix this issue will be the best option

Find the problematic plugin and change:

parent::WP_Widget

To

parent::__construct

I guess you are using some plugin that is not updated after wordpress updates and having some code like class ***_Widget extends WP_Widget { .you should update that plugin or deactivate it until It is updated.