Whenever I add in PHP code in my HTML file, (using MAMP) I got this HTTP ERROR 500 warning

1

I have set up MAMP on my macbook pro.
However right now if I write any sort of PHP file into the htdocs file, or write any sort of PHP code inside my HTML file, I got this error message:

37%20AM

IF I remove my php code, the error disappear.
Where did it go wrong??