Theme_Upgrader::check_package() WordPress Method

	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 does not contain a valid theme header.' ),
					'<code>style.css</code>'
				)
			);
		}

		/*
		 * Parent themes must contain an index file:
		 * - classic themes require /index.php
		 * - block themes require /templates/index.html or block-templates/index.html (deprecated 5.9.0).
		 */
		if (
			empty( $info['Template'] ) &&
			! file_exists( $working_directory . 'index.php' ) &&
			! file_exists( $working_directory . 'templates/index.html' ) &&
			! file_exists( $working_directory . 'block-templates/index.html' )
		) {
			return new WP_Error(
				'incompatible_archive_theme_no_index',
				$this->strings['incompatible_archive'],
				sprintf(
					/* translators: 1: templates/index.html, 2: index.php, 3: Documentation URL, 4: Template, 5: style.css */
					__( 'Template is missing. Standalone themes need to have a %1$s or %2$s template file. <a href="%3$s">Child themes</a> need to have a %4$s header in the %5$s stylesheet.' ),
					'<code>templates/index.html</code>',
					'<code>index.php</code>',
					__( 'https://developer.wordpress.org/themes/advanced-topics/child-themes/' ),
					'<code>Template</code>',
					'<code>style.css</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;
	}