/*Date validation, fixed regex for formats which use dash as separator: added $regexp = preg_replace('/(?<=(\]|\)))'.$separator.'(?=(\[|\())/i', "\\" . $separator, $regexp); to escape dash only between groups. Removed optional two digits year and one digit day or month to fit MySQL date format requirement */

formvalidation.php

function is_valid_date($value, $format = 'dd.mm.yyyy'){
    if(strlen($value) >= 6 && strlen($format) == 10){

        // find separator. Remove all other characters from $format
        $separator_only = str_replace(array('m','d','y'),'', $format);
        $separator = $separator_only[0]; // separator is first character

        if($separator && strlen($separator_only) == 2){
            // make regex
            $regexp = str_replace('mm', '(0[1-9]|1[0-2])', $format);
            $regexp = str_replace('dd', '(0[1-9]|[1-2][0-9]|3[0-1])', $regexp);
            $regexp = str_replace('yyyy', '(19|20)[0-9][0-9]', $regexp);
            $regexp = preg_replace('/(?<=(\]|\)))'.$separator.'(?=(\[|\())/i', "\\" . $separator, $regexp);
            if($regexp != $value && preg_match('/'.$regexp.'\z/', $value)){

                // check date
                $arr=explode($separator,$value);
                $day= intval($arr[2]);
                $month= intval($arr[1]);
                $year=intval($arr[0]);

                return checkdate($month, $day, $year);
            }
        }
    }
    return false;
}