WordPress: Import Image (and set as Featured image) from an ACF custom field URL: custom_field_name

set-featured-images.php

// Import Image and set as Featured image from an ACF custom field URL: custom_field_name 
// Import is done when the post is visited and logged in as admin
// so you can process and test one by one

function does_file_exists($filename) {
    global $wpdb;
    $data = $wpdb->get_results( "SELECT post_id FROM {$wpdb->postmeta} WHERE meta_value LIKE '%/$filename'" );
    foreach ($data as $ID){
        $ID = $ID->post_id;
        if (get_post_type($ID) == "attachment"){
            return $ID;
        }
    }
    return null;
}

add_action("template_redirect",function(){
  if(!current_user_can('administrator')) return;
	if( /*isset($_GET) && isset($_GET['ii']) && */ is_single()) {
		global $post; 
		set_featured_image  ($post -> ID);
		wp_die(" set_featured_image completed");
	}
});

function upload_image_from_URL($image_url, $post_id){
    // Add Featured Image to Post
    $image_name       = pathinfo($image_url)['basename'];
    $upload_dir       = wp_upload_dir(); // Set upload folder
    $image_data       = file_get_contents($image_url); // Get image data
    $unique_file_name = wp_unique_filename( $upload_dir['path'], $image_name ); // Generate unique name
    $filename         = basename( $unique_file_name ); // Create image file name

    // Check folder permission and define file location
    if( wp_mkdir_p( $upload_dir['path'] ) ) {
        $file = $upload_dir['path'] . '/' . $filename;
    } else {
        $file = $upload_dir['basedir'] . '/' . $filename;
    }

    // Create the image  file on the server
    file_put_contents( $file, $image_data );

    // Check image file type
    $wp_filetype = wp_check_filetype( $filename, null );

    // Set attachment data
    $attachment = array(
        'post_mime_type' => $wp_filetype['type'],
        'post_title'     => sanitize_file_name( $filename ),
        'post_content'   => '',
        'post_status'    => 'inherit'
    );

    // Create the attachment
    $attach_id = wp_insert_attachment( $attachment, $file, $post_id );

    // Include image.php
    require_once(ABSPATH . 'wp-admin/includes/image.php');

    // Define attachment metadata
    $attach_data = wp_generate_attachment_metadata( $attach_id, $file );

    // Assign metadata to attachment
    wp_update_attachment_metadata( $attach_id, $attach_data );
    return $attach_id;
}

function set_featured_image($post_id){
    if (wp_is_post_revision($post_id)) return;
    if (has_post_thumbnail($post_id)) return;

    $image_url =  get_field('custom_field_name', $post_id);
 	
	//wp_die($image_url);
    
	if ($image_url){        
		
        $attachment_ID = does_file_exists(pathinfo($image_url)['basename']);
        $attachment_ID = ($attachment_ID ? $attachment_ID : upload_image_from_URL($image_url, $post_id));

        if ($attachment_ID){
            $message = "Setting attachment ". $attachment_ID ." to post ". $post_id;
            error_log($message);
            set_post_thumbnail($post_id, $attachment_ID);
        }
    }
}