Issabel will work only with PHP 5.4. We do not know how did you upgrade your php version, for that same reason it is not possible to give you exact commands, but the base is this:
uninstall php7 with something like: "yum remove php7", you might need to remove more than one package.
disable repository from where php7 was installed
install php, with "yum install php", you might need to install more packages
yum install php
yum install php-pear
yum install php-cli
yum install php-gd
yum install php-pear-DB
yum install php-Smarty
etc..
