set a chosen image-file-input as a featured image from wp frontend

Question

Re: set the FIRST image input to be set as a featured-image/post-thumbnail

Given that you know input name="image-one" is going to be your featured image, you can check for image-one as your $file_handler value in function insert_attachment after the call to media_handle_upload, then set the thumbnail. See the following code.

Do note that I set image-one as the field name to look for and pass it every time.

<?php
$thumbnail_field = 'image-one';
if ( ! empty( $_FILES ) ) {
    foreach ( $_FILES as $file => $array )
        $newupload = insert_attachment( $file, $pid, $thumbnail_field );
}

//attachment helper function   
function insert_attachment( $file_handler, $post_id, $set_thumb = false ) {
    if ( UPLOAD_ERR_OK !== $_FILES[ $file_handler ]['error'] )
        return false; 

    require_once ABSPATH . 'wp-admin/includes/image.php';
    require_once ABSPATH . 'wp-admin/includes/file.php';
    require_once ABSPATH . 'wp-admin/includes/media.php';

    $attach_id = media_handle_upload( $file_handler, $post_id );

    //set post thumbnail (featured)
    if ( $attach_id && $set_thumb )
        update_post_meta( $post_id, '_thumbnail_id', $attach_id );

    return $attach_id;
}
?>

Re: assign any chosen image-input-field as a featured-image

I think this part isn’t needed as you should just label the first image field as “Featured Image”. It’s possible the submitter doesn’t want to set a featured image now.

In looking at the code above, replace line $thumbnail_field = 'image-one'; with the following.

if ( ! empty( $_POST[ 'thumbnail-field' ] ) )
    $thumbnail_field = esc_html( $_POST[ 'thumbnail-field' ] );
else
    $thumbnail_field = 'image-one';

This references a select field like below. Radio’s and jQuery could be used as well to format and populate.

<label for="thumbnail-field">
    Thumbnail field
    <select name="thumbnail-field">
        <option value="image-one">1st</option>
        <option value="image-two">2nd</option>
        <option value="image-three">3rd</option>
    </select>
</label>

Lastly, you might also refresh your memory on PHP’s POST method uploads and check out WordPress’s coding standards for writing more readable code.

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