Chapter 1. STEP 1 :

Install Remi Repository and EPEL Repository on your server :

Section 1. RHEL 7

wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
wget http://rpms.remirepo.net/enterprise/remi-release-7.rpm
rpm -Uvh remi-release-7.rpm epel-release-latest-7.noarch.rpm
subscription-manager repos --enable=rhel-7-server-optional-rpms

Section 2. RHEL 6

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 epel-release-latest-6.noarch.rpm
rhn-channel --add --channel=rhel-$(uname -i)-server-optional-6

Section 3. CentOS 7

wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
wget http://rpms.remirepo.net/enterprise/remi-release-7.rpm
rpm -Uvh remi-release-7.rpm epel-release-latest-7.noarch.rpm

Section 4. CentOS 6

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 epel-release-latest-6.noarch.rpm

Chapter 2. STEP 2 :

After you have installed repository you need to follow some extra configurations :

Section 1. Run this command one by one for RHEL or CentOS

yum install yum-utils
yum-config-manager --enable remi-php72

Chapter 3.  STEP 3 :

Now just run this command :

yum update -y

You’ll see php packages are getting updated, After the update process you need to restart Apache server/php-fpm-(nginx) and check the php version from by below mentioned command:

php -v

[root@server ~]# php -v
PHP 7.2.7 (cli) (built: Jun 20 2018 08:21:26) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.2.7, Copyright (c) 1999-2018, by Zend Technologies

Troubleshoot

If php upgrade process fails (no update/nothing happens) or you upgraded previously from 5.6 to 7.0/7.1 then you need to run this commands to disable php 5.6/7.0/7.1 :

yum-config-manager --disable remi-php56
yum-config-manager --disable remi-php70
yum-config-manager --disable remi-php71

After that follow the tutorial from STEP 2 again.