Syntax Highlighting is where you get some meaningful colors to indicate what you are doing.
for example, in this forum if you use the [ PHP ] tags, you get color coding to tell you functions, variables, or just plain text .
Example Using the PHP tag
$result = mysql_query('SELECT * from TABLE');
// The above is a function to show highlighting.
You can see the difference in the Syntax Highlighting and how it makes it easier to read.
Personally, I use VI in linux and it does it for me.
This is a preference. I find the Syntax Highlighting helps me find errors. (A missed quote, or imporperly formed variable, etc…) But I also use notepad to do some editing too.
OpenOffice.ORG has a FREE suite of Software to rival Microsoft Office (all but databasing). IN there the editor will also do Syntax Highlighting if you save a file as the PHP extension.