Posting a form with mysqli - php

Question

I've created a web page working with php and mysqli in which I can send and edit MySQL entries without errors. The entries have title and date. The problem, appears when I try to change a date that is NULL.
Create an entry with date: Works
Create an entry without date: Works
Edit an entry with a date: Works
Edit an entry with a date to NULL: Works
Edit an entry with date NULL to an existing date: (doesn't work)
I've tried and tested things and I can assume that the problem is that the "day", "month" and "year" inputs are in blank/empty when the form is generated by PHP.
Also can't understand why the form to create an entry, which is exactly the same but with all the inputs in blank at the beginning, works in every case.
This is how I generate my form:
$mysqli->set_charset('Latin1');
$id=$_POST["id"];
$res = $mysqli->query("SELECT * FROM `list` WHERE `id`=$id");
while($row = $res->fetch_assoc()) {
if ($row["fecha"]==NULL) {$day="";$month="";$year="";}else{list($year, $month, $day) = explode("-", $row["date"]);};
echo "
<div class='home'>
<form accept-charset='Latin1' method='post' action='http://mypage.com/?opt=list&f=view'>
<input type='text' value='".$row['title']."' name='title' id='title' required><br>
<input type='number' value=".$day." name='day' id='day' min='1' max='31' placeholder='day'>
<input type='number' value=".$month." name='month' id='month' min='1' max='12' placeholder='month'>
<input type='number' value=".$year." name='year' id='year' placeholder='year'>
<input style='display:none;' type='number' value=".$id." name='id' id='id' placeholder='id'>
<input type='button' id='datepicker'><br>
<input type='submit' name='edit' value='Update'>
</form>
</div>";
};
And this is what happens when I submit:
$edit=$_POST["edit"];
if ($edit) {
$id=$_POST["id"];
$title=$_POST["title"];
if (empty($_POST["year"])&&empty($_POST["month"])&&empty($_POST["day"])){
$res = $mysqli->query("UPDATE `list` SET `title`=('$title'),`date`=(NULL) WHERE `id` = $id;");
}else{
$date=$_POST["year"]."-".$_POST["month"]."-".$_POST["day"];
$res = $mysqli->query("UPDATE `list` SET `title`=('$title'),`date`=('$date') WHERE `id` = $id;");
};
print ("Entry updated to ".$date); //When it doesn't works nothing appears in $date
};
I don't have any kind of idea of why it doesn't work JUST when the "day", "month" and "year" inputs are empty at the beginning.
PS. This is a solution (when date is NULL give values and then clear them when the form is generated), but I still don't know what is the problem with the inputs:
...
if ($row["fecha"]==NULL) {$day="0";$month="0";$year="0";}else{list($year, $month, $day) = explode("-", $row["date"]);};
echo "...";
if ($row["fecha"]==NULL){echo "<script>document.getElementById('day').value='';document.getElementById('month').value='';document.getElementById('year').value='';</script>";
...
Thanks to any help.

Answer 1

When you submit form, empty values from inputs are not submitted as null. They are submitted as empty string "" or zero in case of input which type is number.
Best way to avoid this is to use empty() function:
if (empty($_POST["year"]) && empty($_POST["month"]) && empty($_POST["day"])) {
// code ...
}
This function returns true when the variable is null, zero, false or empty.
Updated answer:
If you can't edit null date column to some date, make sure that:
That the column type of DATE
That your variables that contain year, month and day are formatted with precending zeros
Wrong examples: day = 5, month = 9
Good examples: day = '05', month = '09'
When you concate them as integers you get '2016-9-5' which is invalid date format.
So you should format them as strings with precending zeros like this '2016-09-05'.
Change this line of code:
$date=$_POST["year"]."-".$_POST["month"]."-".$_POST["day"];
To:
$date=sprintf("%04d-%02d-%02d", $_POST["year"], $_POST["month"], $_POST["day"]);

Answer 2

(Posted on behalf of the OP).
The problem was just syntax errors:
value= ' ".$day." '