Could I see concrete steps for building the PHP wrappers on Linux?

Similarly, here is how you would build Python wrappers against the 32-bit PDFNet:

mkdir wrappers_build # Make a directory to build the wrappers in.
cd wrappers_build # Move to that directory.
git clone https://github.com/PDFTron/PDFNetWrappers # Git the code.
cd PDFNetWrappers/PDFNetC # Move to where we download PDFNet.
wget http://www.pdftron.com/downloads/PDFNetC.tar.gz # Download PDFNet.
tar xzvf PDFNetC.tar.gz # Unpack PDFNet.
mv PDFNetC/Headers/ . # Move PDFNet Headers/ into place.
mv PDFNetC/Lib/ . # Move PDFNet Lib/ into place.
cd … # Go back up.
mkdir Build # Create a directory to create the Makefiles in.
cd Build # Move to that directory.
cmake -D BUILD_PDFNetPython=ON … # Create the Makefiles with CMake.
make # Build the Python wrappers with SWIG.
make install # Copy the Python wrappers to where the samples can find them.
cd …/Samples # Move to the Samples directory.
./runall_python.sh # Run all Python code samples, using the new wrappers.

[Note that building UCS2 wrappers and UCS4 wrappers are identical processes, as long as CMake is able to find the correct header and configuration of that Python variant.]