Although the @amzn-main repo
doesn't have PHP 7.2 yet (as far as I know), you can use remi-php72. According to his release blog you can install the EPEL and Remi repositories via:
wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
wget http://rpms.remirepo.net/enterprise/remi-release-6.rpm
rpm -Uvh remi-release-6.rpm
rpm -Uvh epel-release-latest-6.noarch.rpm
And then enable remi-php72 using yum-config-manager:
yum install yum-utils
yum-config-manager --enable remi-php72
After that, you can simply search and install php and all the needed extensions like normal:
sudo yum install --disablerepo="*" --enablerepo="remi,remi-php72" php php-fpm php-cli ...