Using filter_var() to verify date?

$myregex = '~^\d{2}/\d{2}/\d{4}$~';

The regex matched because you just require that pattern anywhere in the string. What you want is only that pattern and nothing else. So add ^ and $.

Note that this still doesn't mean the value is a valid date. 99/99/9999 will pass that test. I'd use:

if (!DateTime::createFromFormat('d/m/Y', $string))

Using regex to validate date is a bad idea .. Imagine 99/99/9999 can easily be seen as a valid date .. you should checkdate

bool checkdate ( int $month , int $day , int $year )

Simple Usage

$date = "01/02/0000";
$date = date_parse($date); // or date_parse_from_format("d/m/Y", $date);
if (checkdate($date['month'], $date['day'], $date['year'])) {
    // Valid Date
}

A simple and convenient way to verify the date in PHP is a strtotime function. You can validate the date with only one line:

strtotime("bad 01/02/2012 bad"); //false
strtotime("01/02/2012"); //1325455200 unix timestamp