vCard download response

<?php

namespace App\Contacts;

use Symfony\Component\HttpFoundation\Response;

final class VcardBuilder
{
    /** @param array{first:string,last:string,org?:string,title?:string,phone?:string,email?:string,url?:string,note?:string} $c */
    public static function build(array $c): string
    {
        $lines = [
            'BEGIN:VCARD',
            'VERSION:3.0',
            'N:'.self::esc($c['last']).';'.self::esc($c['first']).';;;',
            'FN:'.self::esc(trim($c['first'].' '.$c['last'])),
        ];

        // Optional properties are skipped when empty so the card stays clean.
        $optional = [
            'ORG' => $c['org'] ?? null,
            'TITLE' => $c['title'] ?? null,
            'TEL;TYPE=CELL' => $c['phone'] ?? null,
            'EMAIL;TYPE=INTERNET' => $c['email'] ?? null,
            'URL' => $c['url'] ?? null,
            'NOTE' => $c['note'] ?? null,
        ];
        foreach ($optional as $prop => $value) {
            if (filled($value)) {
                $lines[] = $prop.':'.self::esc((string) $value);
            }
        }
        $lines[] = 'REV:'.gmdate('Ymd\THis\Z');
        $lines[] = 'END:VCARD';

        return implode("\r\n", array_map([self::class, 'fold'], $lines))."\r\n";
    }

    public static function download(array $contact): Response
    {
        $name = preg_replace('/[^a-z0-9]+/i', '-', $contact['first'].'-'.$contact['last']);

        return response(self::build($contact), 200, [
            'Content-Type' => 'text/vcard; charset=utf-8',
            'Content-Disposition' => 'attachment; filename="'.trim($name, '-').'.vcf"',
        ]);
    }

    /** Escape the characters that carry meaning in a vCard value. */
    private static function esc(string $v): string
    {
        return str_replace(["\\", ";", ",", "\r\n", "\n"], ["\\\\", "\;", "\\,", "\\n", "\\n"], $v);
    }

    /** Lines over 75 octets are folded: CRLF plus one space continues the line. */
    private static function fold(string $line): string
    {
        $out = '';
        while (strlen($line) > 75) {
            $cut = 75;
            while ($cut > 0 && (ord($line[$cut]) & 0xC0) === 0x80) {
                $cut--; // never split a multi-byte UTF-8 character
            }
            $out .= substr($line, 0, $cut)."\r\n ";
            $line = substr($line, $cut);
        }

        return $out.$line;
    }
}