php7.3-soap 安装问题，Ubuntu 16.04.6 LTS (Xenial Xerus)

Question

我的系统：Ubuntu 16.04.6 LTS（Xenial Xerus）

我试过：

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudp apt-get install php7.3-soap

错误：

Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package php7.3-soap
E: Couldn't find any package by glob 'php7.3-soap'
E: Couldn't find any package by regex 'php7.3-soap'

为什么？

php -v                          
PHP Warning:  PHP Startup: Unable to load dynamic library 'soap' (tried: /usr/lib/php/20180731/soap (/usr/lib/php/20180731/soap: cannot open shared object file: No such file or directory), /usr/lib/php/20180731/soap.so (/usr/lib/php/20180731/soap.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
PHP 7.3.13-1+ubuntu16.04.1+deb.sury.org+1 (cli) (built: Dec 18 2019 14:48:32) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.13, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.3.13-1+ubuntu16.04.1+deb.sury.org+1, Copyright (c) 1999-2018, by Zend Technologies

问题是什么？

ls /etc/php
5.6  7.0  7.1  7.2  7.3  7.4

解决办法是什么？