I had this issue with Eclipse Mars for PHP developers, 64 bit edition for Windows. I now discovered that highlighting works out-of-the-box with the 32 bit version. Even with a fresh download of the equivalent 64 bit build, highlighting does not work. So I will switch back to 32 bit (this is actually not the first problem I observe with Eclipse 64 bit).
Edit:
I thought this was the solution, so I installed the 32 bit version in C:\Program Files (x86) and set a shortcut in the "Start" menu. When I started Eclipse from there, highlighting again ceased to work. I now got it working again by deleting the .metadata directory in the workspace (i.e. resetting the workspace settings) and re-importing the projects.