I suggest you try my software iCoder. This special software was first made by me two years ago. Many improvements have been made over the years.
Why you should try iCoder:
1. As you said, notepad doesnt show code syntax, iCoder does, it also support many other scripts like HTML, Javascript, ASP, JSP
2. It comes with complete original php help, so it is very easy to look up a function's description.
3. It allows you to make some super fast coding. It can translate special key strokes as a combination to a pre entered text. for example you enter ife, it will automatically insert if () {} else {}.
http://www.eport.webhoster.co.uk/iCoder/
find it out yourself.