Problem with duplicator

Hi to all!
I’m trying to duplicate my site, I have my site online and I want to take it to my MAMP because of some changes that I have in my mind and I want to preview these changes in a sandbox before upload it on the web.
I’ve downloaded Duplicator plugin and followed the how to, so I have my package and installer.php, I put them in my localhost directory that is /home/myname/Documents/MAMP/htdocs/mydupesite
I point it in a browser that is https://localhost:8888/mydupesite/installer.php and I fill in the blanks the settings required (Mysql host, user id, password and so on).
I accept the conditions and click on the GO button, the installer.php says to me that the step 1 is working in progress and a completion bar goes on…. when the operation is completed instead of pass to the step 2 my browser downloads the installer.php and I can’t go on…
Some one can help me?
Thanks