Why is there an underscore for data\_?

Is there a simpler way of writing this, so that it makes more sense in terms of code readability?

My analysis:

data\_ is supposed to take whatever data\_ is, request a message via the variable `postmessage` from the form and make sure it's text and concatenate all this:

```
 $filename = 'data_'.$_REQUEST['postmessage'].'.txt';

```

I am a beginner and my analysis of this code could be wrong, but I am willing to learn from my mistakes.