http://php.net/manual/en/function.strtotime.php
The function expects to be given a string containing an English date format and will try to parse that format into a Unix timestamp (the number of seconds since January 1 1970 00:00:00 UTC), relative to the timestamp given in now, or the current time if now is not supplied.
EDIT
I believe you will achieve what you are looking for with the dateTime class
http://www.php.net/manual/en/class.datetime.php
EDIT 2
As @Mark Baker commented, the most correct function to use is DateTime::createFromFormat
as you can see in this link:
http://www.php.net/manual/en/datetime.createfromformat.php