wp_generate_featured_image.php

<?php
function wp_generate_featured_image( $image_url, $post_id  )
{
    $upload_dir = wp_upload_dir();
    $image_data = __get_remote_content($image_url);
    $filename = basename($image_url);

    if(wp_mkdir_p($upload_dir['path'])){
        $file = $upload_dir['path'] . '/' . $filename;
    }else{
        $file = $upload_dir['basedir'] . '/' . $filename;
    }

    file_put_contents($file, $image_data);

    $wp_file_type = wp_check_filetype($filename, null );

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

    $attach_id = wp_insert_attachment( $attachment, $file, $post_id );

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

    $attach_data = wp_generate_attachment_metadata( $attach_id, $file );

    $res1 = wp_update_attachment_metadata( $attach_id, $attach_data );

    $res2 = set_post_thumbnail( $post_id, $attach_id );
}