ZTS: [Z]end [T]hread [S]afety.
ZTS is a compile time option that cannot be enabled at runtime. It allows the PHP interpreter, which usually executes in a single thread, to be executed in many, each with their own isolated instance of the interpreter.
The only option for you appears to be a fresh build, and then using package building tools for your distribution.
I'm not able to advise on the creation of a deb directly, however, creating an rpm is quite trivial, https://github.com/krakjoe/spex there's a starting place for that, you can then use alien to turn an RPM into a deb if you are not able to find out how to create deb packages directly.
Building from source is going to be inescapable, unless you can find some repository with a thread safe build of PHP, with a complete build environment. To that end, the information http://pthreads.org/building may be of use to you. It really isn't that hard to build PHP, nor does it take many hours, on modern hardware you can have a build in less than a minute. You can and should take the opportunity to trim the fat from your installation.
On a side note, the article you reference about PHP not being thread safe is from 2008, it's very wrong. The core is thread safe, there are a few extensions that are inherently unsafe ( not abstracted badly, but fundamentally unsuitable ).
