how to input default value into hidden input - php

Question

I have this database with the following data:
data 1: text1
data 2: text2
.
.
.
data 14: text 14
I was trying to input this data into a "select" column. So I had done this:
<script>
function dropdownlistchange(dropDown) {
var selectedValue = dropDown.options[dropDown.selectedIndex].value;
document.getElementById("category").value = selectedValue;
}
</script>
and the display is:
<?php
$con= mysqli_connect("localhost","root","") or die ("could not connect to mysql");
mysqli_select_db($con,"adminsys") or die ("no database");
$query = "SELECT * FROM productmodelcategory";
$results = mysqli_query($con, $query)or die("Connection could not be established");
echo "<select class='categoryoption' name='categorylist' id='categorylist' onChange='dropdownlistchange(this);'>";
while ($row = mysqli_fetch_assoc($results))
{
echo "<option value='".$row['Category']."'selected='selected'>".$row['Category']."</option>";
}
echo "</select>";
?>
and my display output is:
<input name="category" type="hidden" id="category" value="" required="required"/>
The problem that I having is I am attaching this coding in a submit form, and when I try to not selecting any value due to leave it as default value, and when i submit form, the value displaying "Column 'Category' cannot be null". Anyone can help me solve this problem? I not sure changing the "onchange" method into what method.

Answer 1

Your category input will hold some value when user actually select something from dropdown. If user didn't select anything in this case category input will not hold any value.
Do one thing assign value attribute to your category input like below
<input name="category" type="hidden" id="category" required="required" value="0"/>
Else, you can set 1st row value as default as follow
<?php
$con= mysqli_connect("localhost","root","") or die ("could not connect to mysql");
mysqli_select_db($con,"adminsys") or die ("no database");
$query = "SELECT * FROM productmodelcategory";
$results = mysqli_query($con, $query)or die("Connection could not be established");
echo "<select class='categoryoption' name='categorylist' id='categorylist' onChange='dropdownlistchange(this);'>";
$i = 0;
$default_value =0;
while ($row = mysqli_fetch_assoc($results))
{
if($i == 0){
$default_value = $row['Category'];
$i = 1;
}
echo "<option value='".$row['Category']."'selected='selected'>".$row['Category']."</option>";
}
echo "</select>";
?>
<input name="category" type="hidden" id="category" required="required" value="<?php echo $default_value;?>"/>