Smart App Download Link

// The short address to share, for example on posters, QR codes and email signatures.
$wpide_app_link_path = '/app/';
// Store pages of your app. Leave a value empty to skip that platform.
$wpide_app_link_ios     = ''; // e.g. https://apps.apple.com/app/id0000000000
$wpide_app_link_android = ''; // e.g. https://play.google.com/store/apps/details?id=com.example.app
// Where everyone else goes (desktop browsers); empty = let WordPress show the page at that address.
$wpide_app_link_other = '';

add_action( 'template_redirect', function () use ( $wpide_app_link_path, $wpide_app_link_ios, $wpide_app_link_android, $wpide_app_link_other ) {
    if ( '' === $wpide_app_link_ios && '' === $wpide_app_link_android ) {
        return;
    }

    $path = isset( $_SERVER['REQUEST_URI'] ) ? (string) wp_parse_url( wp_unslash( $_SERVER['REQUEST_URI'] ), PHP_URL_PATH ) : '';
    $want = (string) wp_parse_url( home_url( $wpide_app_link_path ), PHP_URL_PATH );

    if ( '' === $path || strtolower( untrailingslashit( $path ) ) !== strtolower( untrailingslashit( $want ) ) ) {
        return;
    }

    $agent  = isset( $_SERVER['HTTP_USER_AGENT'] ) ? sanitize_text_field( wp_unslash( $_SERVER['HTTP_USER_AGENT'] ) ) : '';
    $target = $wpide_app_link_other;

    if ( preg_match( '/iPhone|iPad|iPod/i', $agent ) && '' !== $wpide_app_link_ios ) {
        $target = $wpide_app_link_ios;
    } elseif ( false !== stripos( $agent, 'Android' ) && '' !== $wpide_app_link_android ) {
        $target = $wpide_app_link_android;
    }

    if ( '' === $target ) {
        return;
    }

    nocache_headers();
    header( 'Vary: User-Agent', false );
    wp_redirect( esc_url_raw( $target ), 302 ); // phpcs:ignore WordPress.Security.SafeRedirect -- store links set by the site owner.
    exit;
}, 1 );