preg_match: check birthday format (dd/mm/yyyy)

I would suggest using checkdate() for this instead:

if (preg_match("/([0-9]{2})\/([0-9]{2})\/([0-9]{4})/", $cnt_birthday, $matches)) {
    if (!checkdate($matches[2], $matches[1], $matches[3])) {
        $error = true;
        echo '<error elementid="cnt_birthday" message="BIRTHDAY - Please enter a valid date in the format - dd/mm/yyyy"/>';
    }
} else {
    $error = true;
    echo '<error elementid="cnt_birthday" message="BIRTHDAY - Only this birthday format - dd/mm/yyyy - is accepted."/>';
}

So regexp validates the format, checkdate validates the actual date.

Based on Tim's checkdate based solution:

The extraction of day, month and year can easily be done using explode as:

list($dd,$mm,$yyyy) = explode('/',$cnt_birthday);
if (!checkdate($mm,$dd,$yyyy)) {
        $error = true;
}