1. First check whether there is an older version
yum list installed | grep php
2. If you have installed, remove the old version
yum remove php.x86_64 php-cli.x86_64 php-common.x86_64 php-gd.x86_64 php-ldap.x86_64 php-mbstring.x86_64 php-mcrypt.x86_64 php-mysql.x86_64 php-pdo.x86_64
3. Install at the file, if the file does not exist, it is to upgrade, open the site with a browser, you can find the latest version by category.
Installation epel:
rpm -ivh https://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/e/epel-release-7-12.noarch.rpm
Installation php7
rpm -Uvh http://mirror.webtatic.com/yum/el7/webtatic-release.rpm
yum install php70w
Install extensions
yum install php70w-mbstring.x86_64