What are some best practices for converting user-input dates in "d.m.Y" format to UNIX timestamps in PHP?

When converting user-input dates in "d.m.Y" format to UNIX timestamps in PHP, it is important to first validate the input format to ensure it matches the expected format. Once validated, the date can be converted to a UNIX timestamp using the `strtotime()` function in PHP.

$user_input_date = "25.12.2021";
$date_format = "d.m.Y";

// Validate user input date format
if (DateTime::createFromFormat($date_format, $user_input_date) !== false) {
    $unix_timestamp = strtotime($user_input_date);
    echo $unix_timestamp;
} else {
    echo "Invalid date format. Please enter date in d.m.Y format.";
}