I think GPL causes a lot of problems regarding code such as PHP as "including" the file can effectively be considered making the code part of the software. So I think your interpretaton of the license is not correct, that as long as we don't modify htmLawed we can incorporate it in a proprietary system.
"You cannot incorporate GPL-covered software in a proprietary system. The goal of the GPL is to grant everyone the freedom to copy, redistribute, understand, and modify a program. If you could incorporate GPL-covered software into a non-free system, it would have the effect of making the GPL-covered software non-free too. "
If you want to achieve the goal you just mentioned above, I would suggest releasing the code either under the Apache (preferred) or LGPL (still has some issues) license.
In general GPL type licences really don't work well with commercial software.
I'd love to hear your thoughts on this.