Disclaimer, before starting this I do not like ioncube, it is great to protect your source code, but a big pain to setup this extension since it has to be done manually. Further, fi you do a PHP update ioncube may no longer work, causing important sites or services to not work (I wish authors would take another approach).
1.) Download from here: https://www.ioncube.com/loaders.php
2.) Extract the contents and copy the appropriate .so file somewhere.
If you want to be sure of what file to use, you can use the load wizard php file and it will tell you exactly which one to use and copy.
3.) Edit /etc/php.ini and add the following line (of course edit the path to suit your situation):
zend_extension = /usr/lib64/php/modules/ioncube_loader_lin_7.0.so
Errors from the loader wizard:
ionCube installation : missing zend_extension
This means you didn't add the zend_extension line above in step#3 to your php.ini file