How do I make php files preview the text?

In thumbnail view most of my text base files preview the text in them. I love that. My php files don't. I assume there is a setting somewhere to change this. Can someone point me to it? Here is an example of what I am talking about.
phpQuestion

What's the text on the first line of the php file?

Oh, it is blank. I Just assumed it had data in it. Sorry, I usually work in my editor, but I was setting up a new project and it didn't occur to me that the files were blank.