We have successfully developed the larger part of the helper library for Swift, including function calls (with return value, arguments, and output), exceptions, array helper class (native) and native array functions.
The reason we did this now was to try out a "Java to Swift" converter (https://github.com/eyob--/j2swift). The converter was a great help, but the process was painstaking to get that last 1% of syntax differences correct. One very interesting difference between the Swift and Java implementations is nearly all of the code that for Java JNI was written in C/C++, we were able to write in Swift directly. The direct interface with PHP is still written in C++, and we added a small wrapper written in C to bridge that to Swift.
We won't be immediately going forward with a Swift converter, as there are still improvements to be made on the Java side, and because Swift is still developing as a language. Maybe when Swift 4 comes, there will be some demand for fully automated "PHP - Swift" converter.