您可以使用wp\\u insert\\u attachment（）函数提供的示例codex page 和set\\u post\\u缩略图（）too :

$file = $_FILES[\'thumbnail\'][\'tmp_name\'];

/*
 * $filename is the path to your uploaded file (for example as set in the $_FILE posted file array)
 * $file is the name of the file
 * first we need to upload the file into the wp upload folder.
 */
$upload_file = wp_upload_bits( $filename, null, @file_get_contents( $file ) );
if ( ! $upload_file[\'error\'] ) { 
    // Check the type of file. We\'ll use this as the \'post_mime_type\'.
    $filetype = wp_check_filetype( $filename, null );

    // Get the path to the upload directory.
    $wp_upload_dir = wp_upload_dir();

    // Prepare an array of post data for the attachment.
    $attachment = array( 
        \'post_mime_type\' => $filetype[\'type\'],
        \'post_title\'     => preg_replace( \'/\\.[^.]+$/\', \'\', $filename ),
        \'post_content\'   => \'\',
        \'post_status\'    => \'inherit\',
        \'post_parent\'    => $post_id
    );

    // Insert the attachment.
    $attach_id = wp_insert_attachment( $attachment, $upload_file[\'file\'], $post_id );

    if ( ! is_wp_error( $attachment_id ) ) { 
        // Make sure that this file is included, as wp_generate_attachment_metadata() depends on it.
        require_once( ABSPATH . \'wp-admin/includes/image.php\' );

        // Generate the metadata for the attachment, and update the database record.
        $attach_data = wp_generate_attachment_metadata( $attach_id, $upload_file[\'file\'] );
        wp_update_attachment_metadata( $attach_id, $attach_data );

        set_post_thumbnail( $post_id, $attach_id );
    }
}

似乎有一些不必要的代码。请尝试以下方法：

$title   = $_POST[\'title\'];
$content = $_POST[\'content\'];

# Set Post

    $new_post = array(
        \'post_title\'    => $title,
        \'post_content\'  => $content,
        \'post_status\'   => \'pending\',
        \'post_type\'     => \'post\',
    );

    $post_id = wp_insert_post($new_post);

    # Set Thumbnail

  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(\'thumbnail\', $post_id);

if (!is_wp_error($attachment_id)) { 
        set_post_thumbnail($post_id, $attachment_id);
        header("Location: " . $_SERVER[\'HTTP_REFERER\'] . "/?files=uploaded");
    }