First make sure that the date you've inputted is in a date form and not a string by using the strtotime function. So....
$DateValue = "6/31/02";
$newDateValue = strtotime($DateValue);
After that use the mktime function to convert this to a timestamp. So it'd probably look something like this...
$var = mktime(0, 0, 0, date("m", $newDateValue), date("d", $newDateValue), date("Y", $newDateValue));
The first 3 zeros are the for hour, minute and second in that order. I'm guessing you don't just want the date in there and not the time so those are set to zero. The mktime function should convert it to July 1. When you output the variable make sure you format it from a timestamp to a regular date
$newVar = date("m/d/Y", $var);
print("$newVar");
or something like that.
For more info...
http://www.php.net/manual/en/function.mktime.php
Hope this helps.