How to set uploaded image as featured image wordpress

Question

How do I set an uploaded image from form to be the featured image of the post.

Here are the basics of my code:

User fills form and uploads image. Submitted form data appears as a custom post type. Image uploaded would be featured image of custom post.

Code

<form method='post' enctype='multipart/form-data'>
    First Name: <input type='text' name='fname'>
    Message: <textarea name='message'></textarea>
    Photo: <input type='file' name='file'>
    <input type='submit' name='submit'>
</form>

<?php if(isset($_POST['submit']) && !empty($_POST)) {
//Upload Image
    require_once( ABSPATH . 'wp-admin/includes/image.php' );
    require_once( ABSPATH . 'wp-admin/includes/file.php' );
    require_once( ABSPATH . 'wp-admin/includes/media.php' );
    $attachment_id = media_handle_upload('file',30);
    if (is_wp_error($attachment_id)) { 
        echo 'BIG TIME ERROR'; } 
    else { 
        echo 'BIG TIME SUCCESS';}

//Insert Post
    $my_post = array(
    'post_title'    => $_POST['fname'],
    'post_content'  => $_POST['message'],
    'post_status'   => 'draft',
    'post_author'   => 1,
    'post_category' => array(8,39),
    'post_type'      => 'my_custom_post'
    );
    wp_insert_post( $my_post );

//Set Image as thumbnail
    set_post_thumbnail($my_post, $attachment_id); //Don't know what to do with this
?>

I have done research and it looks like the function that does that is set_post_thumbnail or update_post_meta?

I'll appreciate any answers with extra information

Answer 1

Did you read doc for set_post_thumbnail()?

You need either post object or post ID for the set_post_thumbnail() function. The function wp_insert_post() will return post id on success. You can use that post id to set the thumbnail image.

Here is the modification for your last couple of lines

$post_id = wp_insert_post( $my_post );

if(!$post_id){
    // post insertion was failed. Handle it here
}

//Set Image as thumbnail
set_post_thumbnail($post_id, $attachment_id); //Don't know what to do with this

That should do it.

code not tested