I think a bigger concern than an exploit would be the possibility that another employee within the organization, who has access to view the source code, might take advantage of the keys to do something unwanted.

In every company I've worked for, there's been much less stringent security applied to viewing scripts than to other activities like logging in to databases, launching scripts to live websites, etc. In the corporate world, lazy server administrators often give temps and low-level employees access to much more information than they need or should be able to see.

Before you even worry about some hypothetical software flaw whereby a user might view your script over the Internet, think carefully about restricting the set of people in your company who might have unfettered access to whatever keys or other sensitive information you're hoping to conceal.

--Kevin