tl;dr
How to solve version \'GLIBCXX_3.4.15\' not found
when I cannot be root on the Linux server?
I\'m tring to use Phan
Since OpenShift Online runs on RHEL, you should be able to install CentOS into a VM on your local machine (or run one in the cloud somewhere) and compile a version of PhantomJS with the patch and then upload it to your OpenShift server and use it.
If you can't do that please use the contact us form at help.openshift.com and reference this stackoverflow question.