Counting field and update database

[phpnewbie112]

Hi everyone, I am new to php and I was trying hard to alter an existing script ut I didn't succeed to...

I have 2 databases:
- Categories (catid, shortname, description)
- Transactions (date, amount, balance, user)

in the categories I have 9 categories.

the script displays the categories as checkboxes to choose from. I want to add the following functionality:

If I select for example 3 categories, I would like to know how can I count the selected checkboxes and based on the count update the transactions database:

in another way:
if count=3 => add amount x 3 to the transactions database so if a user select 4 categories I update the transaction database with $5 x 4 categories and then we will get a balance of $20 for the choosen categories.

I have tried to use:

$checked = count($_POST['category']);

but with this one, if I already have 2 selected then choose another two then the database will update 4 cats rather then the new added 2.

thanks in advance

[assimlation]

you can get a list/count of selected checkboxes like this:

<?php
 
$categories = $_POST['categories'];
 
if (isset($categories))
{
    foreach ($categories as $value)
    {
        echo "$value<br>";
    }
 
    echo "<Br><BR>Items:";
    echo sizeof($categories);
}
else
{
    echo ("<form method=post>
    <input type='checkbox' name='categories[]' value='Comedy'>Comedy<BR>
    <input type='checkbox' name='categories[]' value='Horror'>Horror<BR>
    <input type='checkbox' name='categories[]' value='Action'>Action<BR>
    <input type='checkbox' name='categories[]' value='Musical'>Musical<BR>
    <input type='Submit' value='Select'>");
}
 
?>