📦 WordPress : Set first image as featured image

functions.php

// Get the first embedded image from the content
function catch_the_first_image() {
    global $post, $posts;
    $first_img = '';
    ob_start();
    ob_end_clean();
    $output = preg_match_all('/<img.+?src=[\'"]([^\'"]+)[\'"].*?>/i', $post->post_content, $matches);
    
    if ( isset( $matches[1][0] ) ) {
        $first_img = $matches[1][0];
    }

    return $first_img;
}

// Set auto featured image
function set_featured_image() {
    global $post;
    $post_id = $post->ID;
                 
    if ( strlen( catch_the_first_image() ) >= 5 && get_the_post_thumbnail( $post_id ) == '' ) {
    $image_url = get_site_url() . catch_the_first_image();
    
    $arrContextOptions = array(
        "ssl" => array(
            "verify_peer"=>false,
            "verify_peer_name"=>false,
        ),
    );
    
    
    $upload_dir = wp_upload_dir();
    
    if ( is_ssl() ) {
        $image_data = file_get_contents( $image_url, false, stream_context_create( $arrContextOptions ) );
    }
    else {
        $image_data = file_get_contents( $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_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 );
    }
}
// Use it temporary to generate all featured images
add_action('the_post', 'set_featured_image');