Php – DateTime::createFromFormat('Ymd', '19992323') allows incorrect date

PHP’s DateTime functions and class methods are somewhat forgiving and are not meant to be used for strict validation. However, given you just have a simple year, month, and day smashed together, it is trivial to write a validation function:

$inputDateString = '19992323';

$date = DateTime::createFromFormat(
    'Ymd',
    filter_var(
        $inputDateString,
        FILTER_CALLBACK,
        [
            'options' => function($date) {
                // Does the input string have the correct date format?
                if(!preg_match('/^(?<Y>[0-9]{4})(?<m>[0-9]{2})(?<d>[0-9]{2})$/', $date, $parts)) {
                    return false;
                }
                // Is the year within a given range?
                // Current range is between 0 and the current server year.
                if(
                    !filter_var(
                        $parts['Y'],
                        FILTER_VALIDATE_INT,
                        [
                            'options' => [
                                // Minimum year
                                'min_range' => 0,
                                // Maximum year
                                'max_range' => date('Y')
                            ]
                        ]
                    )
                ) {
                    return false;
                }
                // Is the month between 1 and 12?
                if(
                    !filter_var(
                        $parts['m'],
                        FILTER_VALIDATE_INT,
                        [
                            'options' => [
                                // January
                                'min_range' => 1,
                                // December
                                'max_range' => 12
                            ]
                        ]
                    )
                ) {
                    return false;
                }
                $lastDayOfMonth =
                    (DateTime::createFromFormat('Ymd', $parts['Y'] . $parts['m'] . '01'))
                        ->modify('last day of this month');
                // Is the day between 1 and the last day of the
                //     given month and year?
                if(
                    !filter_var(
                        $parts['d'],
                        FILTER_VALIDATE_INT,
                        [
                            'options' => [
                                // First day of month
                                'min_range' => 1,
                                // Last day of month
                                'max_range' => $lastDayOfMonth->format('d')
                            ]
                        ]
                    )
                ) {
                    return false;
                }
                return $date;
            }
        ]
    )
);

var_dump($date); // $date is false since month is greater than 12
// $date would be a DateTime object if the input date string were valid.