Posts: Auto Set First Image as Featured - WP / Code Snippets Pro

auto_set_featured.php

<?php // <- remove this line if you copy/paste into Code Snippets Pro

/**
 * Posts: Auto Set First Image as Featured
 *
 * Takes the first image attached to a post and sets it as the post's featured image.
 * 
 * Code from: https://wordpress.stackexchange.com/a/131436
 */
namespace xKceqd2;
// from: https://wordpress.stackexchange.com/a/131436
function auto_set_featured($post = NULL)
{
    // retrieve post object
    $post = get_post($post);
	
    // nothing to do if no post, or post already has thumbnail
    if (!$post instanceof \WP_Post || has_post_thumbnail($post->ID))
        return;

	// prepare $thumbnail var
    $thumbnail = NULL;

    $args = array(
        'post_parent' => $post->ID,
        'post_type' => 'attachment',
        'post_status' => 'inherit',
		'post_mime_type' => 'image',
		'order' => 'ASC',
		'numberposts' => 1,
        'posts_per_page' => 1
    );
	
	// retrieve all the images uploaded to the post
    $images = get_posts($args);
    
	// if we got some images, save the first in $thumbnail var
    if (is_array($images) && !empty($images))
        $thumbnail = reset($images);
    
	// if $thumbnail var is valid, set as featured for the post
    if ($thumbnail instanceof \WP_Post)
        set_post_thumbnail($post->ID, $thumbnail->ID);
}

add_action('save_post', '\xKceqd2\auto_set_featured');

PHP snippet for Code Snippets Pro
Posts: Auto Set First Image as Featured
Takes the first image attached to a post and sets it as the post's featured image.
Code from: https://wordpress.stackexchange.com/a/131436