how to get a sum of the value which i get in the if condition

first of all i am new in this programming area so i don't know how to do this?

have a database table "video_master" in that i have "ID, REC_ID, VIDEO_DATE, VIDEO_COUNT, IS_REDIM, field in it. now i want a count of that amount. i use if and get a amount as video but now i don't know how to take a that amount sum. total amount. here i give what i done.

<?php

            // If user not logged in then show login message
            if (!isset($_SESSION['userid'])) {

                ?><div class="message">
                    To access this page, you must be logged.<br />
                    <a href="connexion.php">Log in</a>
                </div><?php
            }
            else{

                //Video count values of the database
                if(isset($_REQUEST['date1']) && isset($_REQUEST['date2'])){
                    $dnn = mysqli_query($conn, 'select video_date,video_count from video_master where  rec_id="'.$_SESSION['userid'].'" and video_date >= "' . $_REQUEST['date1'] . '" and video_date <= "' . $_REQUEST['date2'] . '"');
                }
                else{
                    $dnn = mysqli_query($conn, 'select id,video_date,video_count,is_redim from video_master where  rec_id="'.$_SESSION['userid'].'"');

                 }


                if(mysqli_num_rows($dnn)>0)
                {



                    ?><table>
    <thead>
                        <tr>
                            <th>ID</th>
                            <th>Date</th>
                            <th>Video Count</th>

                            <th>Total Amount</th>
                            <th>Is Redim ?</th>
                        </tr></thead><?php

                        while($rowData = mysqli_fetch_array($dnn)){
                            //$video_date = date("Y-m-d","d-m-Y",$rowData['video_date']);
                            $video_date = $rowData['video_date'];
                            $video_count = $rowData['video_count'];

                            ?><tbody><tr>
                                <td><?php echo $rowData['id']; ?></td>
                                <td><?php print($video_date); ?></td>
                                <td><?php print($video_count); ?></td>
                                <td><?php //echo $rowData['video_count'];

                                    if($rowData['video_count'] < 250){
                          $amount1 = $rowData['video_count']*0;
                          echo $amount1;            
                } 
                else{
                            if($rowData['video_count'] <= 500){
                            $amount1 = $rowData['video_count']*0.25;
                            echo $amount1;            
                            } else
                            {
                                if($rowData['video_count'] <= 750){
                                $amount1 = $rowData['video_count']*0.50;
                                echo $amount1;            
                                } else
                                {
                                    if($rowData['video_count'] <= 1000){
                                    $amount1 = $rowData['video_count']*0.75;
                                    echo $amount1;            
                                    } else
                                    {
                                            if($rowData['video_count'] <= 1250){
                                            $amount1 = $rowData['video_count']*1;
                                            echo $amount1;            
                                            } else
                                            {

                                                if($rowData['video_count'] < 1500){
                                                $amount1 = $rowData['video_count']*1.50;
                                                echo $amount1;            
                                                } else
                                                {



                                                }        


                                            }        

                                    }


                                }                      

                            } 

                    } 


                                 ?></td>
                                <td><?php echo $rowData['is_redim'];  ?></td>
                            </tr></tbody><?php

                        }
                    ?></table><?php
                }   



                ?>
            <?php

            }


    ?>

i get a "$amount" as a video but now i want to calculate the total of that $amount. amount field is not in the database. i get the amount from video_count.