PHP表单验证提交问题

Every time I try to submit the form and I have not entered nothing in the year field I get Incorrect year! how can I still submit the form without having to enter a year. In other words leaving the year field blank and not getting a warning?

Here is the PHP code.

if(preg_match('/^\d{4,}$/', $_POST['year'])) {
    $year = mysqli_real_escape_string($mysqli, $_POST['year']);
} else {
    $year = NULL;
}

if($year == NULL) {
    echo '<p class="error">Incorrect year!</p>';
} else {
    //do something
}

if(preg_match('/^\d{4,}$/', $_POST['year'])) {
    $year = mysqli_real_escape_string($mysqli, $_POST['year']);
} else if(empty($_POST['year'])){
    $year = '';
} else {
    $year = null;
} 

if($year == NULL) {
    echo '<p class="error">Incorrect year!</p>';
} else {
    //do something
}

Perhaps I'm misunderstanding, but this sounds like what you want.

if(preg_match('/^\d{4,}$/', $_POST['year'])) {
    $year = mysqli_real_escape_string($mysqli, $_POST['year']);
} else {
    $year = false;
}

if ($year === false) {
    echo '<p class="error">Incorrect year!</p>';
} else {
    //do something
}

OR (if you didn't set YEAR somewhere else)

if(preg_match('/^\d{4,}$/', $_POST['year'])) {
    $year = mysqli_real_escape_string($mysqli, $_POST['year']);
}

if (!isset($year)) {
     echo '<p class="error">Incorrect year!</p>';
} else {
    //do something
}

Hey you can just take this part out and it will take the echo out allowing you to submit with the field being blank

if($year == NULL) {
    echo '<p class="error">Incorrect year!</p>';
} else {
    //do something
}

I wouldn't suggest allowing it to go blank into the database. Maybe you can just grab the default date on the server or a default value meaning the user didn't fill it out. This would take the error out but still leave something in the database field.