如何在PHP中实现DES加密，使其与已有的解密函数兼容？

public function hextostr(string $hex) {
    $string = "";
    for ($i = 0; $i < strlen($hex) - 1; $i += 2) {
        $string .= chr(hexdec($hex[$i] . $hex[$i + 1]));
    }
    return $string;
}

public function desDecrypt(string $hex, string $key) {
    $base64 = base64_decode($this->hextostr($hex));
    return openssl_decrypt($base64, 'des-ecb', $key);
}

class DES {
    private $key;

    public function __construct(string $key) {
        $this->key = $key;
    }

    private function strToHex(string $string): string {
        $hex = '';
        for ($i = 0; $i < strlen($string); $i++) {
            $hex .= dechex(ord($string[$i]));
        }
        return $hex;
    }

    public function desEncrypt(string $string): string {
        $encrypted = openssl_encrypt($string, 'des-ecb', $this->key);
        $base64 = base64_encode($encrypted);
        return $this->strToHex($base64);
    }

    public function desDecrypt(string $hex): string {
        $base64 = base64_decode($this->hextostr($hex));
        return openssl_decrypt($base64, 'des-ecb', $this->key);
    }
}

$key = "testkey";
$des = new DES($key);
$data = "123";
$encryptedData = $des->desEncrypt($data);
echo "加密: " . $encryptedData . "\n";
$decryptedData = $des->desDecrypt($encryptedData);
echo "解密: " . $decryptedData . "\n";