public function check_package( $source ) {
    global $wp_filesystem, $wp_version;

    $this->new_theme_data = array();

    if ( is_wp_error( $source ) ) {
      return $source;
    }

    // Check that the folder contains a valid theme.
    $working_directory = str_replace( $wp_filesystem->wp_content_dir(), trailingslashit( WP_CONTENT_DIR ), $source );
    if ( ! is_dir( $working_directory ) ) { // Sanity check, if the above fails, let's not prevent installation.
      return $source;
    }

    // A proper archive should have a style.css file in the single subdirectory.
    if ( ! file_exists( $working_directory . 'style.css' ) ) {
      return new WP_Error(
        'incompatible_archive_theme_no_style',
        $this->strings['incompatible_archive'],
        sprintf(
          /* translators: %s: style.css */
          __( 'The theme is missing the %s stylesheet.' ),
          '<code>style.css</code>'
        )
      );
    }

    // All these headers are needed on Theme_Installer_Skin::do_overwrite().
    $info = get_file_data(
      $working_directory . 'style.css',
      array(
        'Name'        => 'Theme Name',
        'Version'     => 'Version',
        'Author'      => 'Author',
        'Template'    => 'Template',
        'RequiresWP'  => 'Requires at least',
        'RequiresPHP' => 'Requires PHP',
      )
    );

    if ( empty( $info['Name'] ) ) {
      return new WP_Error(
        'incompatible_archive_theme_no_name',
        $this->strings['incompatible_archive'],
        sprintf(
          /* translators: %s: style.css */
          __( 'The %s stylesheet doesn&#8217;t contain a valid theme header.' ),
          '<code>style.css</code>'
        )
      );
    }

    // If it's not a child theme, it must have at least an index.php to be legit.
    if ( empty( $info['Template'] ) && ! file_exists( $working_directory . 'index.php' ) ) {
      return new WP_Error(
        'incompatible_archive_theme_no_index',
        $this->strings['incompatible_archive'],
        sprintf(
          /* translators: %s: index.php */
          __( 'The theme is missing the %s file.' ),
          '<code>index.php</code>'
        )
      );
    }

    $requires_php = isset( $info['RequiresPHP'] ) ? $info['RequiresPHP'] : null;
    $requires_wp  = isset( $info['RequiresWP'] ) ? $info['RequiresWP'] : null;

    if ( ! is_php_version_compatible( $requires_php ) ) {
      $error = sprintf(
        /* translators: 1: Current PHP version, 2: Version required by the uploaded theme. */
        __( 'The PHP version on your server is %1$s, however the uploaded theme requires %2$s.' ),
        phpversion(),
        $requires_php
      );

      return new WP_Error( 'incompatible_php_required_version', $this->strings['incompatible_archive'], $error );
    }
    if ( ! is_wp_version_compatible( $requires_wp ) ) {
      $error = sprintf(
        /* translators: 1: Current WordPress version, 2: Version required by the uploaded theme. */
        __( 'Your WordPress version is %1$s, however the uploaded theme requires %2$s.' ),
        $wp_version,
        $requires_wp
      );

      return new WP_Error( 'incompatible_wp_required_version', $this->strings['incompatible_archive'], $error );
    }

    $this->new_theme_data = $info;

    return $source;
  }