首先看自己php的版本是多少

root@xx:~# php -v
PHP 7.0.33-0ubuntu0.16.04.2 (cli) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.0.33-0ubuntu0.16.04.2, Copyright (c) 1999-2017, by Zend Technologies

ok 是7.0，那麼我們要升級至7.3，因為laravel最新版要求的是7.3，所以我只好升級至7.3

步驟如下：

sudo apt-get install software-properties-common
sudo add-apt-repository ppa:ondrej/php

sudo apt update

sudo apt-get install php7.3 libapache2-mod-php7.3 php7.3-cli php7.3-mysql php7.3-gd php7.3-imagick php7.3-recode php7.3-tidy php7.3-xmlrpc

最後查一下是否真正升級成7.3了？

root@xx:~# php -v
PHP 7.3.3-1+ubuntu16.04.1+deb.sury.org+1 (cli) (built: Mar  7 2019 20:31:26) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.3, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.3.3-1+ubuntu16.04.1+deb.sury.org+1, Copyright (c) 1999-2018, by Zend Technologies

ok，確實升級成7.3了