<?php
// r10.net/iskendercnr
require __DIR__ . '/wp-load.php';

$args = array(
    'post_type' => 'post',
    'posts_per_page' => -1,
);

$posts = get_posts($args);

foreach ($posts as $post) {
    $post_id = $post->ID;
    echo "Konu: " . $post->post_title;

    $attachments = get_posts(array(
        'post_type' => 'attachment',
        'posts_per_page' => -1,
        'post_parent' => $post_id,
    ));
    if ($attachments) {
        echo "<br>Resim ID: " . $attachments[0]->ID;
        set_post_thumbnail($post_id, $attachments[0]->ID);
        echo "<br>işlem tamam: " . $post_id . "<br>";
    }

}

?>