Cacher is the code snippet organizer for pro developers

アイキャッチ画像のURLを出力する関数

the_post_thumbnail_url

// アイキャッチ画像のURL文字列を出力する
the_post_thumbnail_url( $size );

アイキャッチ画像のURLを取得する関数

get_the_post_thumbnail_url( get_the_ID(), $size );

ソースコードを確認

/**
 * 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 );
    }
}