For Downgrading 7.2 (default) to 7.1 PHP
sudo add-apt-repository ppa:ondrej/php sudo apt-get update sudo apt-get install libapache2-mod-php7.1 php7.1 php7.1-bcmath php7.1-bz2 php7.1-cgi php7.1-cli php7.1-common php7.1-curl php7.1-dba php7.1-dev php7.1-enchant php7.1-fpm php7.1-gd php7.1-gmp php7.1-imap php7.1-interbase php7.1-intl php7.1-json php7.1-ldap php7.1-mbstring php7.1-mcrypt php7.1-mysql php7.1-odbc php7.1-opcache php7.1-pgsql php7.1-phpdbg php7.1-pspell php7.1-readline php7.1-recode php7.1-snmp php7.1-soap php7.1-sqlite3 php7.1-sybase php7.1-tidy php7.1-xml php7.1-xmlrpc php7.1-xsl php7.1-zip sudo a2dismod php7.2 sudo a2enmod php7.1 sudo service apache2 restart sudo update-alternatives --set php /usr/bin/php7.1
Reference: Sandy B @ mysterydata.com