Change hidden field value on Submit click - php

Question

I have a form with multiple items and an id attributed to each of these items, on Submit I want to be able to grab the id of the item that was clicked - I have tried using js, something like:
<form method="post" action="add_item_cart.php">
<input type="hidden" id="item_id" name="item_id">
<input name="submit_item" id="btn_sub" onclick="document.getElementById('item_id').value = <?php echo '3'; ?>" type="submit" value="Add">
</form>
I want to be able to grab this value: $item_id = $_POST["item_id"]; on add_item_cart.php, but this doesn't seem to be working.
Is this a problem with my js syntax or is my logic not plausible to solve this problem? Is it submitting before changing the value?
EDIT:
Let's see if I can explain myself better, I want to assign that hidden value dynamically, imagine that my form has 3 submit buttons (one for each item displayed). Depending on the one that is clicked, I want to pass the item's id to my hidden field, so if I click button1 - $_POST["item_id"]=1, button2 - $_POST["item_id"]=2... etc
Here is my actual form (non simplified example)
<form method="post" action="add_item_cart.php">
<table style="width:600px">
<tr>
<?php foreach ($items as $item): ?>
<td>
<center>
<span style="font-size:20px"><?php echo $item["item_name"] ?></span><br/>
€<?php echo $item["price"] ?><br/>
Quantidade: <input type="text" value="1" style="width:30px"><br/>
<input type="hidden" id="item_id" name="item_id">
<input name="submit_item" id="btn_sub" onclick="document.getElementById('item_id').value = <?php echo $item["id"]; ?>" type="submit" value="Adicionar">
</center>
</td>
<?php endforeach; ?>
</tr>
</table>
</form>

Answer 1

When your form is posted and you want to collect the item_id value on add_item_cart.php first you need to actually assign a value to id as in (Assuming item_id is a php variable). The id is just used for setting the css editing not a value...
<input type="hidden" id="item_id" value="<?php echo $item_id; ?>" name="item_id">
you cannot have id='' for the value because 'value' is value.
Then you can get that value on your other page with:
<?php
if(isset($_POST['item_id'])){
$item_id = $_POST['item_id'];
}
?>
If you want to edit the variable after post depending on which button you hit you can try.
<input name="submit_item1" id="btn_sub" name="button1" type="submit" value="Add">
<input name="submit_item2" id="btn_sub" name="button1" type="submit" value="Add">
<input name="submit_item3" id="btn_sub" name="button1" type="submit" value="Add">
Then on the top of your page you can do.
<?php
if(isset($_POST['submit_item1'])){
$item_id = 1;
}
if(isset($_POST['submit_item2'])){
$item_id = 2;
}
if(isset($_POST['submit_item3'])){
$item_id = 3;
}
?>
If you are creating forms from an array of items you could do something like, (assuming you somehow have an id associated with those items; I would need to know more information about how you are making your item list. But it would generally do like this.
<?php
foreach($item_array as $item){
?>
<form method="post" action="add_item_cart.php">
<input type="hidden" id="item_id" name="item_id" value="<?php echo $item['id']; ?>">
<input name="submit_item" id="btn_sub" type="submit" value="Add">
</form>
<?php
}
?>
Then on the top of your page you can just get $_POST['item_id'] and since that value is dynamically set you do not need any conditionals you can just get that value and run any query.

Answer 2

UPDATE
Use normal button instead of submit button and use javascript for submitting the form.
<form method="post" name="f1" action="add_item_cart.php">
<input type="hidden" id="item_id" name="item_id">
<input name="submit_item" id="btn_sub" onclick="document.getElementById('item_id').value = <?php echo '3'; ?>; document.f1.submit();" type="button" value="Add">