<?php

function CheckAdditional($value)
{
    return date('Y-m-d', strtotime($value)) == $value;
}

echo var_dump(CheckAdditional('moo!'))."\n";
echo var_dump(CheckAdditional('2012-02-31'))."\n";
echo var_dump(CheckAdditional('2012-a-4'))."\n";
echo var_dump(CheckAdditional('2012-06-13'))."\n";
?>

bool(false)

bool(false)

bool(false)

bool(true)