Instead of modifying the sidebar or templates itself, why not create a plugin with a widget that does just what you need. This way
you don't hard code and keeps thing dynamic. This will also allow you to move the widget around at will
you keep your templates clean, simple and maintainable
do it the correct way. Stuff like widgets, registering custom post types and taxonomies and shortcodes should always be registered in a plugin and not inside a theme
I have written a very basic widget template a while ago which you can check out here. We can use this as a basis to create our custom widget. I will comment the parts where you an modify the output. You can also alter the widget as needed.
Lets look at the plugin (simply create a file in your plugins folder, add this code and activate the plugin)
NOTE: All code is untested and might be buggy. Also, you will need to have PHP 5.4+ installed for this to work
<?php
/*
Plugin Name: Blog Page Featured Image
Plugin URI: https://wordpress.stackexchange.com/q/219344/31545
Description: Displays the blog page's feaured image in the sidear
Version: 1.0.0
Author: Pieter Goosen
Author URI: https://wordpress.stackexchange.com/users/31545/pieter-goosen
License: GPL2
License URI: https://www.gnu.org/licenses/gpl-2.0.html
*/
class Blog_Page_Featured_Image extends WP_Widget
{
public function __construct()
{
parent::__construct(
'widget_blog_page_featured_image',
_x( 'Blog Page Featured Image', 'Blog page featured image' ),
[ 'description' => __( 'Displays the featured image for the pge set as blog page.' ) ]
);
$this->alt_option_name = 'widget_blog_page_featured_image';
add_action( 'save_post', [$this, 'flush_widget_cache'] );
add_action( 'deleted_post', [$this, 'flush_widget_cache'] );
add_action( 'switch_theme', [$this, 'flush_widget_cache'] );
}
public function widget( $args, $instance )
{
$cache = [];
if ( ! $this->is_preview() ) {
$cache = wp_cache_get( 'widget_bpfi', 'widget' );
}
if ( ! is_array( $cache ) ) {
$cache = [];
}
if ( ! isset( $args['widget_id'] ) ) {
$args['widget_id'] = $this->id;
}
if ( isset( $cache[ $args['widget_id'] ] ) ) {
echo $cache[ $args['widget_id'] ];
return;
}
ob_start();
$title = ( ! empty( $instance['title'] ) ) ? $instance['title'] : __( 'Blog Page Featured Image' );
/** This filter is documented in wp-includes/default-widgets.php */
$title = apply_filters( 'widget_title', $title, $instance, $this->id_base );
// ADD YOUR CUSTOM PHP CODE HERE FOR EXECUTION TO DISPLAY ON FRONT END
// First make sure this is the page set as blogpage
if ( 'page' === get_option('show_on_front')
&& is_home()
) { // We are on the blogpage
// Get the current page object
$page_object = $GLOBALS['wp_the_query']->get_queried_object();
// Make sure that the current page IS the one set as blogpage
if ( $page_object->ID === (int) get_option( 'page_for_posts' ) ) {
/**
* Now that we are sure we are on the posts page, lets get and display
* its featured image. We will also first make sure that we
* actually have one set
*/
if ( has_post_thumbnail( $page_object->ID ) )
echo get_the_post_thumbnail( $page_object->ID );
}
}
echo $args['after_widget'];
if ( ! $this->is_preview() ) {
$cache[ $args['widget_id'] ] = ob_get_flush();
wp_cache_set( 'widget_bpfi', $cache, 'widget' );
} else {
ob_end_flush();
}
}
public function update( $new_instance, $old_instance )
{
$instance = $old_instance;
$instance['title'] = strip_tags( $new_instance['title'] );
$this->flush_widget_cache();
$alloptions = wp_cache_get( 'alloptions', 'options' );
if ( isset($alloptions['widget_blog_page_featured_image']) )
delete_option('widget_blog_page_featured_image');
return $instance;
}
public function flush_widget_cache()
{
wp_cache_delete('widget_bpfi', 'widget');
}
public function form( $instance )
{
$title = isset( $instance['title'] ) ? esc_attr( $instance['title'] ) : '';
?>
<p>
<label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label>
<input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo $title; ?>" />
</p>
<?php
}
}
add_action( 'widgets_init', function ()
{
register_widget( 'Blog_Page_Featured_Image' );
});
What I also like to do is to always completely remove a widget when it is is totally out of context to avoid the sidebar from rendering a blank space when no other widgets are displayed
We can try the same approach as I have described here. You can just add this at the bottom of the plugin
add_filter( 'sidebars_widgets', function ( $sidebars_widgets )
{
// Return our filter when we are on admin screen
if ( is_admin() )
return $sidebars_widgets;
// Make sure we are not on the blog page, if we are, bail
if ( 'page' === get_option('show_on_front')
&& is_home()
&& $GLOBALS['wp_the_query']->get_queried_object_id() === (int) get_option( 'page_for_posts' )
)
return $sidebars_widgets;
/**
* Widget we need to target. This should be the name/id we used to register it
*
* EXAMPLE
* parent::__construct(
'widget_blog_page_featured_image',
_x( 'Blog Page Featured Image', 'Blog page featured image' ),
[ 'description' => __( 'Displays the featured image for the pge set as blog page.' ) ]
);
*
*/
$custom_widget = 'widget_blog_page_featured_image';
// See if our custom widget exists is any sidebar, if so, get the array index
foreach ( $sidebars_widgets as $sidebars_key=>$sidebars_widget ) {
// Skip the wp_inactive_widgets set, we do not need them
if ( $sidebars_key == 'wp_inactive_widgets' )
continue;
// Only continue our operation if $sidebars_widget are not an empty array
if ( $sidebars_widget ) {
foreach ( $sidebars_widget as $k=>$v ) {
/**
* Look for our custom widget, if found, unset it from the $sidebars_widgets array
* @see stripos()
*/
if ( stripos( $v, $custom_widget ) !== false )
unset( $sidebars_widgets[$sidebars_key][$k] );
} // endforeach $sidebars_widget
} // endif $sidebars_widget
} // endforeach $sidebars_widgets
return $sidebars_widgets;
});
EDIT
The code above is now tested and working as expected
