https://ahmetimamoglu.com.tr/wordpresste-otomatik-one-cikarilan-gorsel-nasil-belirlenir

Wordpress-Auto-Featured-Image.php

<?php
function auto_featured_image() {
    global $post;

    if ( ! has_post_thumbnail($post->ID) ){
        $attached_image = get_children("post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1");

        if ($attached_image) {
            foreach ($attached_image as $attachment_id => $attachment) {
                set_post_thumbnail($post->ID, $attachment_id);
            }
        }
    }
}

add_action('the_post', 'auto_featured_image');