Insert PHP variable PHP $ _POST

I have a question / problem, I want to insert in a table in my database a value that is PHP variable, when passing it through $ _POST it does not keep it, is there another way to do it ?, I leave the code, maybe I I made a mistake:

                $totalprice=0;  
                foreach($_SESSION['pedido'] as $id => $value) {

                $sql="SELECT * FROM producto WHERE Id IN (";$sql.=$id.",";$sql=substr($sql, 0, -1).") ORDER BY Tipo ASC";
                $query=mysqli_query($bd, $sql);

                while($row=mysqli_fetch_array($query)){ 
                    $subtotal=$_SESSION['pedido'][$row['Id']]['quantity']*$row['Valor']; 

                    $totalprice+=$subtotal; 
                ?> 
                    <tr> 
                        <td><?php echo $row['Descripcion'] ?></td> 
                        <td><input type="text" class="form-control" placeholder="Cantidad" name="quantity[<?php echo $row['Id'] ?>]" size="5" value="<?php echo $_SESSION['pedido'][$row['Id']]['quantity'] ?>" /></td> 
                        <td>$<?php echo $row['Valor'] ?></td> 
                        <td>$<?php echo $_SESSION['pedido'][$row['Id']]['quantity']*$row['Valor'] ?></td> 
                    </tr> 
                <?php 

                }

                } 

                //echo $sql;
                //exit;                     
               ?> 
        <div class="form-group">
                <tr>    
                    **<td colspan="4" name="total"><strong>Total:</strong> <?php echo $totalprice ?></td>**
                </tr> 
                </div>

PS: What I want to save is the value of $ totalprice

When passing it through $ _POST, I should be passing it like this: $ Total = $ _POST [$ totalprice];

Is this correct? Please your answers.