Yes, it’s possible. You would just use regex to parse the HTML to get the image URL, upload that to the uploads folder, and then set it as the featured image.
Like this:
function set_featured_image_from_html( $html, $post_id ) {
// If there are no images in the HTML, return early.
if ( ! preg_match( '/<img\s+.*?src=[\"\']?([^\"\' >]*)[\"\']?[^>]*>/i', $html, $image ) ) {
return;
}
$image_url = $image[1];
$upload_dir = wp_upload_dir();
$image_data = file_get_contents( $image_url );
$filename = basename( $image_url );
if ( wp_mkdir_p( $upload_dir['path'] ) ) {
$file = $upload_dir['path'] . "https://wordpress.stackexchange.com/" . $filename;
} else {
$file = $upload_dir['basedir'] . "https://wordpress.stackexchange.com/" . $filename;
}
file_put_contents( $file, $image_data );
$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'
);
$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 );
wp_update_attachment_metadata( $attach_id, $attach_data );
set_post_thumbnail( $post_id, $attach_id );
}
You would use it like this:
set_featured_image_from_html('<img src="https://picsum.photos/800">', 29);