Probably you have a mess with env paths.
Check files .bashrc or .bash_profile and remove old version path.
Last chance is to use direct/explicitly:
source /opt/rh/devtoolset-7/enable
Answer from Nikola Lukic on Stack Overflowinstall gcc successful, but can't enable - Stack Overflow
Installing centos-release-scl
yum install centos-release-scl-rh failing
Problem with missing yum package CentOS (Virtualmin) - Unix & Linux Stack Exchange
It appears that the PHP 7.1 software collection is not built for CentOS 6, but is available in CentOS 7.
You could try to rebuild the rh-php71 software collection for CentOS 6, but I have not found any information that anyone has tried to do that.
You can find information about available software collections from their website or from the configured YUM repository
When looking for the correct package to install into a CentOS system, the first place I tend to look is under centos.pkgs.org. Their search comes up with mostly exhaustive results for the major .rpm repositories out there as well as instructions on how to install each package on the relevant page.
php71 rpm for CentOS 6 (remi repo)