You can't do it alone with PHP as that information is not available. However, if you know the user's location you may be able to do some geo-locating and determine a probably timezone.
You need javascript's help. I googled the following JS snippet which returns the client's timezone offset:
var tzo=(new Date().gettimezoneoffset()/60)*(-1);
From here, you can set a cookie with JS which is accessible with PHP. Alternativly, you could grab this information with an ajax call.
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