參考網址：https://vitux.com/how-to-install-php5-php8-on-ubuntu/
The official PHP website, php.net, provides a list of all PHP versions to date at the following link:
$ sudo add-apt-repository ppa:ondrej/php
apt install -y php5.6
apt-get install php5.6-gd php5.6-mysql php5.6-imap php5.6-curl php5.6-intl php5.6-pspell php5.6-recode php5.6-sqlite3 php5.6-tidy php5.6-xmlrpc php5.6-xsl php5.6-zip php5.6-mbstring php5.6-soap php5.6-opcache libicu65 php5.6-common php5.6-json php5.6-readline php5.6-xml