Description:
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Lets assume you have a form, and you want the user to input a date. In this 
example, I'm going to use a UK formatted date (tho a US formatted or other format 
of date will work.

Lets say our user enters "01/12/2012" (1st Dec 2012) as their date. They hit 
submit on the form.

Currently if you wish to validate that date, you have to first sanitize it, Then 
to run the checkdate function, it needs the date to be split into month, day and 
year. This is all well and good, but now we've got to explode our date, check the 
exploded content is valid, then run checkdate.

Would it not be simpler to be able to do this:

filter_var($_POST['date'], FILTER_SANITIZE_DATE);

(A FILTER_VALIDATE_DATE) would also be handy here)

This would allow you to strip out having to explode the date string and run 
checkdate.

In the background, this would simply be doing a check to see the date format.

Alternatively, some sort of function to automatically detect a date format, and 
convert it to the gregorian date, which obviously for a date (without a time) is 
the preferable end result given that it will likely be stored in this format in a 
database.