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function is not described

_wp_post_thumbnail_html() WP 2.9.0

Returns HTML for the post thumbnail meta box.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

Return

String. The post thumbnail HTML.

Usage

_wp_post_thumbnail_html( $thumbnail_id, $post );
$thumbnail_id(int)
ID of the attachment used for thumbnail
$post(int|WP_Post)
The post ID or object associated with the thumbnail.
Default: global $post

Changelog

Since 2.9.0 Introduced.

Code of _wp_post_thumbnail_html() WP 5.7.2

<?php
function _wp_post_thumbnail_html( $thumbnail_id = null, $post = null ) {
	$_wp_additional_image_sizes = wp_get_additional_image_sizes();

	$post               = get_post( $post );
	$post_type_object   = get_post_type_object( $post->post_type );
	$set_thumbnail_link = '<p class="hide-if-no-js"><a href="%s" id="set-post-thumbnail"%s class="thickbox">%s</a></p>';
	$upload_iframe_src  = get_upload_iframe_src( 'image', $post->ID );

	$content = sprintf(
		$set_thumbnail_link,
		esc_url( $upload_iframe_src ),
		'', // Empty when there's no featured image set, `aria-describedby` attribute otherwise.
		esc_html( $post_type_object->labels->set_featured_image )
	);

	if ( $thumbnail_id && get_post( $thumbnail_id ) ) {
		$size = isset( $_wp_additional_image_sizes['post-thumbnail'] ) ? 'post-thumbnail' : array( 266, 266 );

		/**
		 * Filters the size used to display the post thumbnail image in the 'Featured image' meta box.
		 *
		 * Note: When a theme adds 'post-thumbnail' support, a special 'post-thumbnail'
		 * image size is registered, which differs from the 'thumbnail' image size
		 * managed via the Settings > Media screen.
		 *
		 * @since 4.4.0
		 *
		 * @param string|int[] $size         Requested image size. Can be any registered image size name, or
		 *                                   an array of width and height values in pixels (in that order).
		 * @param int          $thumbnail_id Post thumbnail attachment ID.
		 * @param WP_Post      $post         The post object associated with the thumbnail.
		 */
		$size = apply_filters( 'admin_post_thumbnail_size', $size, $thumbnail_id, $post );

		$thumbnail_html = wp_get_attachment_image( $thumbnail_id, $size );

		if ( ! empty( $thumbnail_html ) ) {
			$content  = sprintf(
				$set_thumbnail_link,
				esc_url( $upload_iframe_src ),
				' aria-describedby="set-post-thumbnail-desc"',
				$thumbnail_html
			);
			$content .= '<p class="hide-if-no-js howto" id="set-post-thumbnail-desc">' . __( 'Click the image to edit or update' ) . '</p>';
			$content .= '<p class="hide-if-no-js"><a href="#" id="remove-post-thumbnail">' . esc_html( $post_type_object->labels->remove_featured_image ) . '</a></p>';
		}
	}

	$content .= '<input type="hidden" id="_thumbnail_id" name="_thumbnail_id" value="' . esc_attr( $thumbnail_id ? $thumbnail_id : '-1' ) . '" />';

	/**
	 * Filters the admin post thumbnail HTML markup to return.
	 *
	 * @since 2.9.0
	 * @since 3.5.0 Added the `$post_id` parameter.
	 * @since 4.6.0 Added the `$thumbnail_id` parameter.
	 *
	 * @param string   $content      Admin post thumbnail HTML markup.
	 * @param int      $post_id      Post ID.
	 * @param int|null $thumbnail_id Thumbnail attachment ID, or null if there isn't one.
	 */
	return apply_filters( 'admin_post_thumbnail_html', $content, $post->ID, $thumbnail_id );
}