Insert and update different tables in database using php ajax

You made the move to using PDO and creating Prepared Statements but then spoil the good work by embedding variables directly within the SQL commands. I couldn't test any of the following but I hope it helps.

<script>
    <?php
        $data=array(
            'id'                =>  $row->pid,
            'oldstock'          =>  $row->pstock,
            'productcategory'   =>  $row->pcategory,
            'productname'       =>  $row->pname,
            'currentDate'       =>  $savedate
        );
        printf('var json=%s;',json_encode($data));
    ?>
    
    $(document).ready(function() {
        $('#btn_stockin').click(function(event) {   
            event.preventDefault();

            /*Reading value from modal*/
            var newStock = $('#txt_addstock').val();
            var newPrice = $('#txt_addprice').val();
            
            if( newStock == '' && newPrice == '' ){
                alert("Oops!, fill Text fields and try again.");
            }else{
                let args={
                    stock_up:newStock,
                    cost: newPrice
                };
                
                let payload=Object.assign(args,json);
            
                $.ajax({
                    method:'POST', 
                    url:'stock-in.php',
                    data:payload,
                    success:function(data){
                        $('#add_stock_modal').modal('hide');
                        window.location.reload();
                    }
                });
            }
        });
    });
</script>

<?php
    
    #stock-in.php
    /*
        If you are using sessions you need to start a session!
    */
    error_reporting( E_ALL );
    session_start();

    
    
    if( empty( $_SESSION['useremail'] ) OR empty( $_SESSION['role'] ) OR $_SESSION['role']=="Admin" ){
        exit( header('Location: index.php') );
    }

    /*
        Check that all fields that are required in the sql have been submitted
    */
    if( isset( 
            $_POST['stock_up'],
            $_POST['productcategory'],
            $_POST['productname'],
            $_POST['currentDate'],
            $_POST['cost'],
            $_POST['id'],
            $_POST['oldstock']
        ) ){

        try{
            
            include_once 'connectdb.php';

            /*
                When inserting, updating multiple tables there is some sense in using a transaction
                so that if one part fails the db is not littered with orphan records
            */
            $pdo->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
            $pdo->beginTransaction();
            
            
            
            
            $product_category   =   $_POST['productcategory'];
            $product_name       =   $_POST['productname'];
            $current_date       =   $_POST['currentDate'];
            $stockup            =   (int)$_POST['stock_up'];
            $newPrice           =   (int)$_POST['cost'];
            $id                 =   $_POST['id'];
            $oldstock           =   (int)$_POST['oldstock'];

            $new_stock          = $oldstock + $stockup;
            $amount_owed        = $newPrice * $stockup;
            
            
            /*
                The sql command should use placeholders rather than embedded fariables - the names are arbitrary
            */
            $sql='insert into `tbl_stock` ( `category_name`, `product_name`, `stock_in`, `stock_price`, `total_cost`, `stocked_date` ) 
                values 
            ( :cat, :prod, :in, :price, :cost, :date )';
            $stmt=$pdo->prepare( $sql );
            $args=array(
                ':cat'      =>  $product_category,
                ':prod'     =>  $product_name,
                ':in'       =>  $stockup,
                ':price'    =>  $newPrice,
                ':cost'     =>  $amount_owed,
                ':date'     =>  $current_date
            );
            if( !$stmt->execute( $args )  )echo 'stmt#1 failed';
            
            
            
            $sql='update `tbl_product` set `pstock` =:stock, `purchaseprice`=:price where `pid`=:pid';
            $stmt=$pdo->prepare( $sql );
            $args=array(
                ':stock'    =>  $new_stock,
                ':price'    =>  $newPrice,
                ':pid'      =>  $id
            );
            if( !$stmt->execute( $args ) )echo 'stmt#2 failed';
            
            
            
            /*
                If it all went well, commit these statements to the db
            */
            if( !$pdo->commit() )echo 'commit failed';
            
            
        
        }catch( PDOException $e ){
            /*
                Any problems, rollback the transaction and report issues - 
                not necessarily with the full `getMessage()` ~ perhaps just
                'Error!' etc
            */
            $pdo->rollBack();
            
            echo $e->getMessage();
        }
    }
?>