in your code split second argument is wrong .
you need to give the $date variable.

See the following PHP code.

process.php
Code:
<?php
$date=$_POST['dob'];

print $date;
$arr=split("/",$date);   
$dd=$arr[0];
$mm=$arr[1];
$yy=$arr[2];

if(checkdate($mm,$dd,$yy))
{
echo 'Valid Date';
}
else
{
        echo 'Invalid Date';
}
?>