Step 1: Add EPEL and REMI Repository

sudo yum -y install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
sudo yum -y install https://rpms.remirepo.net/enterprise/remi-release-8.rpm

Step 2: Install PHP 7.4 on CentOS 8 / RHEL 8

sudo dnf -y install dnf-utils
sudo dnf module install php:remi-7.4
sudo dnf update
sudo yum install php-xxx