First you will have to create gui(text fields and buttons) to get that details
On this form you will have buttons Save and Cancel.
On click of Save button your form(form where that save button is) should be redirected to a little php script to save that comment entry.After inserting that entry into a database that should redirected user to a HOME page or where ever you want him to go on.
These values you will have to store in a database or in a file.
You have two ways
As omol suggested
one is using mysql database.
Create table in a database
Add Fields with proper datatype
name, email, comment, datetime
and second ways is write that entries in a text file or csv separated by delimiter.