Are you looking to programmatically set the featured image aka post thumbnail in WordPress? Sometimes, you need to write a script that should set a thumbnail for a post, page, or custom post type. In this article, we study how to set the featured image programmatically in WordPress.

WordPress comes with numerous useful built-in features. One of them is the Featured Image. A Featured Image is a representative image for a post, page, or custom post type. This image helps to recognize what the content is about.

In WordPress, one can set the post thumbnail either manually or programmatically. Let’s see both options one by one.

Manually Set the Post Thumbnail

We can assign the featured image to specific posts or pages by clicking on the Set featured Image section. It opens a WordPress media uploader where you either choose an existing image or upload a new one.

It is a straightforward way to set the post thumbnail in WordPress. The same steps have to follow for other posts, pages, and post types.

Set the Featured Image Programmatically

Create HTML Form

Setting the featured image via code requires the HTML form with file input and a submit button. Add the below HTML to your page template.

<form method="post" enctype="multipart/form-data">
    <p>
        <label>Select Image:</label>
        <input type="file" name="image" required />
    </p>
    <input type="hidden" name="image_nonce" value="<?php echo wp_create_nonce('image_nonce'); ?>" />
    <input type="submit" name="upload_file" value="Submit" />
</form>

This code will display the form with file input and the submit button.

If you noticed, I have added the hidden field for a nonce. This nonce helps to protect the form against certain types of misuse, and malicious attacks. We’ll verify this nonce on the server side and process the form only if the nonce is valid.

Server-Side Code

Now, when the user clicks the submit button, it should set a featured image. For this, write the code below in the active themes functions.php file.

function fn_set_featured_image() {
    if ( isset( $_POST['upload_file'] ) && wp_verify_nonce( $_REQUEST['image_nonce'], 'image_nonce' ) ) {

        $upload = wp_upload_bits( $_FILES["image"]["name"], null, file_get_contents( $_FILES["image"]["tmp_name"] ) );

        if ( ! $upload['error'] ) {
            $post_id = 'POST_ID_HERE'; //set post id to which you need to add featured image
            $filename = $upload['file'];
            $wp_filetype = wp_check_filetype( $filename, null );
            $attachment = array(
                'post_mime_type' => $wp_filetype['type'],
                'post_title' => sanitize_file_name( $filename ),
                'post_content' => '',
                'post_status' => 'inherit'
            );

            $attachment_id = wp_insert_attachment( $attachment, $filename, $post_id );

            if ( ! is_wp_error( $attachment_id ) ) {
                require_once(ABSPATH . 'wp-admin/includes/image.php');

                $attachment_data = wp_generate_attachment_metadata( $attachment_id, $filename );
                wp_update_attachment_metadata( $attachment_id, $attachment_data );
                set_post_thumbnail( $post_id, $attachment_id );
            }
        }
    }
}
add_action('init', 'fn_set_featured_image');

Here, I am first verifying a nonce using the wp_verify_nonce() method before executing a code.

For setting a post thumbnail I have used a few methods as follows.

• wp_upload_bits() : It creates a file in the wp-content/upload folder with the given content. The file will be created automatically in the current month folder.
• wp_check_filetype() : Retrieve a file type from the file name.
• sanitize_file_name() : It replaces whitespaces with dashes in the file name.
• wp_insert_attachment() : This method inserts an attachment entry into the database.
• wp_generate_attachment_metadata() : It generates metadata of the image along with a thumbnail and other intermediate sizes of the image. These sizes are defined under Settings => Media Screen.
• wp_update_attachment_metadata() : Update the metadata in the post_meta table.
• set_post_thumbnail() : Sets the featured image for the given post.

Go ahead and give it a try. To verify whether the code works or not, go to the edit screen of a specified post and you should see the thumbnail under the Featured Image section. It means you have successfully set the featured image programmatically.

Related Articles

• Upload Image Using Bulletproof Library in PHP
• How to Register Custom Image Sizes and Resize Existing Images in WordPress
• A Guide to Upload and Compress Images in Laravel

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