get_custom_logo( int $blog_id )

function get_custom_logo( $blog_id = 0 ) {
	$html = '';
	$switched_blog = false;

	if ( is_multisite() && ! empty( $blog_id ) && (int) $blog_id !== get_current_blog_id() ) {
		switch_to_blog( $blog_id );
		$switched_blog = true;
	}

	$custom_logo_id = get_theme_mod( 'custom_logo' );

	// We have a logo. Logo is go.
	if ( $custom_logo_id ) {
		$custom_logo_attr = array(
			'class'    => 'custom-logo',
			'itemprop' => 'logo',
		);

		/*
		 * If the logo alt attribute is empty, get the site title and explicitly
		 * pass it to the attributes used by wp_get_attachment_image().
		 */
		$image_alt = get_post_meta( $custom_logo_id, '_wp_attachment_image_alt', true );
		if ( empty( $image_alt ) ) {
			$custom_logo_attr['alt'] = get_bloginfo( 'name', 'display' );
		}

		/*
		 * If the alt attribute is not empty, there's no need to explicitly pass
		 * it because wp_get_attachment_image() already adds the alt attribute.
		 */
		$html = sprintf( '<a href="%1$s" class="custom-logo-link" rel="home" itemprop="url">%2$s</a>',
			esc_url( home_url( '/' ) ),
			wp_get_attachment_image( $custom_logo_id, 'full', false, $custom_logo_attr )
		);
	}

	// If no logo is set but we're in the Customizer, leave a placeholder (needed for the live preview).
	elseif ( is_customize_preview() ) {
		$html = sprintf( '<a href="%1$s" class="custom-logo-link" style="display:none;"><img class="custom-logo"/></a>',
			esc_url( home_url( '/' ) )
		);
	}

	if ( $switched_blog ) {
		restore_current_blog();
	}

	/**
	 * Filters the custom logo output.
	 *
	 * @since WP-4.5.0
	 * @since WP-4.6.0 Added the `$blog_id` parameter.
	 *
	 * @param string $html    Custom logo HTML output.
	 * @param int    $blog_id ID of the blog to get the custom logo for.
	 */
	return apply_filters( 'get_custom_logo', $html, $blog_id );
}