I think your making this way to complicated.
If the user enters the values year month and day then...
$date = "$month/$day/$year";
Of course this relies upon the user entering they values correctly, putting limits in the HTML form can help here, personally I use the below...
<?php
$day = date ("d");
$month = date ("m");
$year = date ("y");
$date = "$day-$month-$year";
?>
Use www.php.net, and search for date() as in above post for more details, there are so many different 'things' built in, the above...
d is day, in xx format, ie 22 for today
m in month in xx format, ie 01 for today
y is year in xx format, ie 01 for today
You can add in other things to give other dates, another site spits out
Monday 22nd January 2001, time is the same using date () and h, i and s type 'things' sure they got a proper name.
Hope that makes sense\helps.