Probleme mit der Verlinkung

<?php
/**
 * WordPress Post Thumbnail Template Functions.
 *
 * Support for post thumbnails.
 * Theme's functions.php must call add_theme_support( 'post-thumbnails' ) to use these.
 *
 * @package WordPress
 * @subpackage Template
 */




/**
 * Check if post has an image attached.
 *
 * @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`.
 * @return bool Whether the post has an image attached.
 */
function has_post_thumbnail( $post = null ) {
    return (bool) get_post_thumbnail_id( $post );
}




/**
 * Retrieve post thumbnail ID.
 *
 * @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`.
 * @return string|int Post thumbnail ID or empty string.
 */
function get_post_thumbnail_id( $post = null ) {
    $post = get_post( $post );
    if ( ! $post ) {
        return '';
    }
    return get_post_meta( $post->ID, '_thumbnail_id', true );
}




/**
 * Display 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
 *
 * @see get_the_post_thumbnail()
 *
 * @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.
 */
function the_post_thumbnail( $size = 'post-thumbnail', $attr = '' ) {
    echo get_the_post_thumbnail( null, $size, $attr );
}




/**
 * Update cache for thumbnails in the current loop.
 *
 * @since 3.2.0
 *
 * @global WP_Query $wp_query
 *
 * @param WP_Query $wp_query Optional. A WP_Query instance. Defaults to the $wp_query global.
 */
function update_post_thumbnail_cache( $wp_query = null ) {
    if ( ! $wp_query )
        $wp_query = $GLOBALS['wp_query'];




    if ( $wp_query->thumbnails_cached )
        return;




    $thumb_ids = array();
    foreach ( $wp_query->posts as $post ) {
        if ( $id = get_post_thumbnail_id( $post->ID ) )
            $thumb_ids[] = $id;
    }




    if ( ! empty ( $thumb_ids ) ) {
        _prime_post_caches( $thumb_ids, false, true );
    }




    $wp_query->thumbnails_cached = true;
}




/**
 * 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 );




    /**
     * Filter the post thumbnail size.
     *
     * @since 2.9.0
     *
     * @param string|array $size The post thumbnail size. Image size or array of width and height
     *                           values (in that order). Default 'post-thumbnail'.
     */
    $size = apply_filters( 'post_thumbnail_size', $size );




    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 = '';
    }
    /**
     * Filter 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 );
}




/**
 * Return the post thumbnail URL.
 *
 * @since 4.4.0
 *
 * @param int|WP_Post  $post Optional. Post ID or WP_Post object.  Default is global `$post`.
 * @param string|array $size Optional. Registered image size to retrieve the source for or a flat
 *                           array of height and width dimensions. Default 'post-thumbnail'.
 * @return string|false Post thumbnail URL or false if no URL is available.
 */
function get_the_post_thumbnail_url( $post = null, $size = 'post-thumbnail' ) {
    $post_thumbnail_id = get_post_thumbnail_id( $post );
    if ( ! $post_thumbnail_id ) {
        return false;
        
    }
    return wp_get_attachment_image_url( $post_thumbnail_id, $size );
}




/**
 * Display the post thumbnail URL.
 *
 * @since 4.4.0
 *
 * @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'.
 */
function the_post_thumbnail_url( $size = 'post-thumbnail' ) {
    $url = get_the_post_thumbnail_url( null, $size );
    if ( $url ) {
        echo esc_url( $url );
        
    }
}