描述：

获取文章的缩略图，该函数会直接返回带有img标签的缩略图。

用法：

<?php echo get_the_post_thumbnail('', '', 'style=border:0;'); ?>

参数：

$post

（string）默认值：null

文章的ID，或者WP_Post对象

$size

（string）默认值：post-thumbnail

指定要返回的缩略图尺寸

$attr

（string）默认为空

为img标签添加一个属性

返回值：

此函数返回如下结构的img标签：

<img width="400" height="300" src="" class="attachment- size- wp-post-image" alt="" style="border: 0;" />

源文件：

/**
 * Retrieve the post thumbnail.
 *
 * 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.
 *
 * When using the_post_thumbnail() or related functions, the 'post-thumbnail' image
 * size is used by default, though a different size can be specified instead as needed.
 *
 * @since 2.9.0
 * @since 4.4.0 `$post` can be a post ID or WP_Post object.
 *
 * @param int|WP_Post  $post Optional. Post ID or WP_Post object.  Default is global `$post`.
 * @param string|array $size Optional. Image size to use. Accepts any valid image size, or
 *                           an array of width and height values in pixels (in that order).
 *                           Default 'post-thumbnail'.
 * @param string|array $attr Optional. Query string or array of attributes. Default empty.
 * @return string The post thumbnail image tag.
 */
function get_the_post_thumbnail( $post = null, $size = 'post-thumbnail', $attr = '' ) {
	$post = get_post( $post );
	if ( ! $post ) {
		return '';
	}
	$post_thumbnail_id = get_post_thumbnail_id( $post );

	/**
	 * Filters the post thumbnail size.
	 *
	 * @since 2.9.0
	 * @since 4.9.0 Added the `$post_id` parameter.
	 *
	 * @param string|array $size    The post thumbnail size. Image size or array of width and height
	 *                              values (in that order). Default 'post-thumbnail'.
	 * @param int          $post_id The post ID.
	 */
	$size = apply_filters( 'post_thumbnail_size', $size, $post->ID );

	if ( $post_thumbnail_id ) {

		/**
		 * Fires before fetching the post thumbnail HTML.
		 *
		 * Provides "just in time" filtering of all filters in wp_get_attachment_image().
		 *
		 * @since 2.9.0
		 *
		 * @param int          $post_id           The post ID.
		 * @param string       $post_thumbnail_id The post thumbnail ID.
		 * @param string|array $size              The post thumbnail size. Image size or array of width
		 *                                        and height values (in that order). Default 'post-thumbnail'.
		 */
		do_action( 'begin_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );
		if ( in_the_loop() ) {
			update_post_thumbnail_cache();
		}
		$html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );

		/**
		 * Fires after fetching the post thumbnail HTML.
		 *
		 * @since 2.9.0
		 *
		 * @param int          $post_id           The post ID.
		 * @param string       $post_thumbnail_id The post thumbnail ID.
		 * @param string|array $size              The post thumbnail size. Image size or array of width
		 *                                        and height values (in that order). Default 'post-thumbnail'.
		 */
		do_action( 'end_fetch_post_thumbnail_html', $post->ID, $post_thumbnail_id, $size );

	} else {
		$html = '';
	}
	/**
	 * Filters the post thumbnail HTML.
	 *
	 * @since 2.9.0
	 *
	 * @param string       $html              The post thumbnail HTML.
	 * @param int          $post_id           The post ID.
	 * @param string       $post_thumbnail_id The post thumbnail ID.
	 * @param string|array $size              The post thumbnail size. Image size or array of width and height
	 *                                        values (in that order). Default 'post-thumbnail'.
	 * @param string       $attr              Query string of attributes.
	 */
	return apply_filters( 'post_thumbnail_html', $html, $post->ID, $post_thumbnail_id, $size, $attr );
}