Here's how I would go about it.
Default timezone you set in your php script, it's whatever you decide it to be.
Then, for display purposes of your users, you determine what's the timezone of your user. You do this using javascript, since you want the info from client side. Some possible solutions:
pure javascript https://stackoverflow.com/a/1091399/525445
jQuery https://stackoverflow.com/a/5607444/525445
(or even - combine these two approaches - get timezone with pure javascript, and then store that value in session variable, like in the jquery example).
3. Now that your php script knows your user's timezone, you can use it to convert your default timezone, to whatever time they have, using this: