(I just want a specific answer as to why this happens)
I have been learning PHP and MySQL here in StackOverflow and most of the time I get too excited and combine things that I have Learned from here so far. Though I try to understand things directly from the mistakes I make, there are still some things I am not able to solve on my own. The most recent problem is:
I have been working on an example for quite a while and I am constantly getting the below error message after clicking the "Add New Record" button.
Though I have no problems and have not received any errors when I only have 3 declared variables when I added more input tags () to my page, I start to receive these error messages and my insert query couldn't get through.
Below are the errors that show after clicking the "Add New Record" button:
Notice: Undefined index: buyprice in C:\xampp\htdocs\sppap\create.php on line 54
Notice: Undefined index: grosssale in C:\xampp\htdocs\sppap\create.php on line 76
and here is my PHP and relevant HTML:
PHP
<?php
    require_once 'config.php';
    $brand = $generic = $wgtvol = $unit =  $manufacturer = "";
    $buyprice = $sellprice = $grosssale = "";
    $brand_err = $generic_err = $wgtvol_err = $unit_err = $manufacturer_err = $buyprice_err = $sellprice_err = $grosssale_err = ""; 
    if($_SERVER["REQUEST_METHOD"] == "POST"){
        $input_brand = trim($_POST["brand"]);
        if(empty($input_brand)){
            $brand_err = '<b style="color: #960303; font-size: 160%">Please enter the brand name of the product.</b>';
        } else{
            $brand = $input_brand;
        }
        $input_generic = trim($_POST["generic"]);
        if(empty($input_generic)){
            $generic_err = '<b style="color: #960303; font-size: 160%">Please enter the generic name of the product.</b>';
        } else{
            $generic = $input_generic;
        }
        $input_wgtvol = trim($_POST["wgtvol"]);
        if(empty($input_wgtvol)){
            $wgtvol_err = '<b style="color: #960303; font-size: 160%">Please enter the weight or volume of the product.</b>';
        } else{
            $wgtvol = $input_wgtvol;
        }
        $input_manufacturer = trim($_POST["manufacturer"]);
        if(empty($input_manufacturer)){
            $manufacturer_err = '<b style="color: #960303; font-size: 160%">Please enter the manufacturer of the product.</b>';
        } else{
            $manufacturer = $input_manufacturer;
        }
        $input_unit = trim($_POST["unit"]);
        if(empty($input_unit)){
            $unit_err = '<b style="color: #960303; font-size: 160%">Please choose a measurement unit of the product.</b>';
        } else{
            $unit  = $input_unit ;
        }
        $input_buyprice = trim($_POST["buyprice"]);
        if(empty($input_buyprice)){
            $buyprice_err = '<b style="color: #960303; font-size: 160%">Please enter the buy price of the product.</b>';
        } else{
            $buyprice = $input_buyprice;
        }
        $input_sellprice = trim($_POST["sellprice"]);
        if(empty($input_sellprice)){
            $sellprice_err = '<b style="color: #960303; font-size: 160%">Please enter the sell price of the product.</b>';
        } else{
            $sellprice = $input_sellprice;
        }
        $input_grosssale = trim($_POST["grosssale"]);
        if(empty($input_grosssale)){
            $grosssale_err = '<b style="color: #960303; font-size: 160%">This field is suppose to auto-generate after typing the buy price and the sell price.<br>There is no need to manually enter an amount into this field.</b>';
        } else{
            $grosssale = $input_grosssale;
        }
        $generic = $brand = $wgtvol = $unit = $manufacturer = $buyprice = $sellprice = $grosssale = "";
        if(empty($generic_err) && empty($brand_err) && empty($wgtvol_err) && empty($unit_err) && empty($manufacturer_err) && empty($buyprice_err) && empty($sellprice_err) && empty($grosssale_err)){
            $sql = "INSERT INTO productslist (generic, brand, wgtvol, unit, manufacturer, buyprice, sellprice, grosssale) VALUES (?, ?, ?, ?, ?, ?, ?, ?)";
            if($stmt = mysqli_prepare($link, $sql)){
                mysqli_stmt_bind_param($stmt, "ssssssss", $param_generic, $param_brand, $param_wgtvol, $param_unit, $param_manufacturer, $param_buyprice, $param_sellprice, $param_grosssale);
                $param_generic = $input_generic;
                $param_brand = $input_brand;
                $param_wgtvol = $input_wgtvol;
                $param_unit = $input_unit;
                $param_manufacturer = $input_manufacturer;
                $param_buyprice = $input_buyprice;
                $param_sellprice = $input_sellprice;
                $param_grosssale = $input_grosssale;
                if(mysqli_stmt_execute($stmt)){
                    header("location: index.php");
                    exit();
                } else{
                    echo "Something went wrong. Please try again later.";
                }
            }
            mysqli_stmt_close($stmt);
        }
        mysqli_close($link);
    }
?>
HTML
<form class="form-wrapper" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
    <div class="form-group <?php echo (!empty($brand_err)) ? 'has-error' : ''; ?>">
        <label>Brand Name</label>
        <input type="text" name="brand" class="form-control" value="<?php echo $brand; ?>" />
        <span class="help-block"><?php echo $brand_err;?></span>
    </div>
    <div class="form-group <?php echo (!empty($generic_err)) ? 'has-error' : ''; ?>">
        <label>Generic Name</label>
        <input type="text" name="generic" class="form-control" value="<?php echo $generic; ?>" />
        <span class="help-block"><?php echo $generic_err;?></span>
    </div>
    <div class="form-group <?php echo (!empty($wgtvol_err)) ? 'has-error' : ''; ?>">
        <div class="input-group">
            <label class="black-text">Weight / Volume</label>
            <input type="number" step="0.01" maxlength="7" class="form-control digitOnly pull-left" id="wgtvol" name="wgtvol" value="<?php echo $wgtvol; ?>" />
            <label class="sr-only black-text">Unit</label>
            <select  name="unit" class="form-control pull-left" id="unit" type="text"  >
                <option disabled value="">select</option>
                <option value="mg"> mg </option>
                <option value="gram(s)"> gram(s) </option>
                <option value="ml"> ml </option>
                <option value="liter(s)">liter(s)</option>
                <option value="dl"> dl </option>
                <option value="cc"> cc </option>
                <option value="pc(s)"> pc(s) </option>
                <option value="fl oz">fl oz</option>
                <option value="gal">gal</option>
            </select>
        </div>
    </div>
    <div class="form-group <?php echo (!empty($manufacturer_err)) ? 'has-error' : ''; ?>">
        <label>Manufacturer</label>
        <input type="text" name="manufacturer" class="form-control" value="<?php echo $manufacturer; ?>" />
        <span class="help-block"><?php echo $manufacturer_err;?></span>
    </div>
    <div class="form-group <?php echo (!empty($buyprice_err)) ? 'has-error' : ''; ?>">
        <label>Buy Price</label>
        <input type="number" step="0.01" maxlength="7" class="form-control digitOnly name="buyprice" class="form-control" value="<?php echo $buyprice; ?>"  />
        <span class="help-block"><?php echo $buyprice_err;?></span>
    </div>=
    <div class="form-group <?php echo (!empty($sellprice_err)) ? 'has-error' : ''; ?>">
        <label>Sell Price</label>
        <input type="number" step="0.01" maxlength="7" class="form-control digitOnly" name="sellprice" value="<?php echo $sellprice; ?>"  />
        <span class="help-block"><?php echo $sellprice_err;?></span>
    </div>
    <div class="form-group <?php echo (!empty($grosssale_err)) ? 'has-error' : ''; ?>">
        <label>Gross Sale</label>
        <input disabled type="number" maxlength="7" class="form-control digitOnly" name="grosssale" value="<?php echo $grosssale; ?> "  />
        <span class="help-block"><?php echo $grosssale_err;?></span>
    </div>
    <input type="submit" class="btn btn-primary" value="Add New Record" />
    <a href="index.php" class="btn btn-default">Cancel</a>
</form>
 
     
    