Problem with isset() when building a contact form

[zhengzh2]

hi,
I am new here, and I have a problem when I was trying to build a contact form, pretty simple one, you know, just less than ten lines of php code and a basic html form, but when I add if(isset($_POST['submit'])) to the php code, it alarm as the 500 error, when I delete it, it works well, so, what's wrong with this.
I would be appreciate if anyone can help, thanks.

 

[caftpx10]

A 500 error would happen when there's an error bad enough in PHP like if there is bad syntax.