Hi Kurt,
PHP 5.6 was released about 5 years after CMSB v1.36, so there will likely be some compatibility issues as each new PHP version changes some things.
I'd recommend upgrading to the latest v3 version, or if not that, the latest v2 branch version (Version 2.65).
And as far as your clients go, developers often bill for upgrades like this, because it's required to keep up with the recommended security patches provided by PHP. The same would be true for any CMS platform the client was on.
Hope that helps! Let me know any questions.
Dave Edis - Senior Developer
interactivetools.com