毎回アイキャッチ画像を本文とは別に用意するのがめんどくさい_(:3 」∠)_
そんなときにブログ記事内の最初の画像を取得してアイキャッチ画像に設定する方法。
以下をfunctions.phpに追加。

//記事内の最初の画像を取得する
function catch_that_image() {
    global $post, $posts;
    $first_img = '';
    ob_start();
    ob_end_clean();
    $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
    $first_img = $matches [1] [0];
  
if(empty($first_img)){ //Defines a default image
        $first_img = "/images/noimage.jpg";
    }
    return $first_img;
}

function re_register_post_tag_taxonomy() {
global $wp_rewrite;
$rewrite = array(
'slug' => get_option('tag_base') ? get_option('tag_base') : 'tag',
'with_front' => ! get_option('tag_base') || $wp_rewrite->using_index_permalinks(),
'ep_mask' => EP_TAGS,
);
 
$labels = array(
'name' => _x( 'Tags', 'taxonomy general name' ),
'singular_name' => _x( 'Tag', 'taxonomy singular name' ),
'search_items' => __( 'Search Tags' ),
'popular_items' => __( 'Popular Tags' ),
'all_items' => __( 'All Tags' ),
'parent_item' => null,
'parent_item_colon' => null,
'edit_item' => __( 'Edit Tag' ),
'view_item' => __( 'View Tag' ),
'update_item' => __( 'Update Tag' ),
'add_new_item' => __( 'Add New Tag' ),
'new_item_name' => __( 'New Tag Name' ),
'separate_items_with_commas' => __( 'Separate tags with commas' ),
'add_or_remove_items' => __( 'Add or remove tags' ),
'choose_from_most_used' => __( 'Choose from the most used tags' ),
'not_found' => __( 'No tags found.' )
);
 
register_taxonomy( 'post_tag', 'post', array(
'hierarchical' => true,
'query_var' => 'tag',
'rewrite' => $rewrite,
'public' => true,
'show_ui' => true,
'show_admin_column' => true,
'_builtin' => true,
'labels' => $labels
) );
}
add_action( 'init', 're_register_post_tag_taxonomy', 1 );