如何从mysql查询中总结金额

I wrote this code to sum only amount but is not working.Everything works perfectly except the sum. Can some one help me out? Here is the code:

<tr >
 <td > Total </td>
 <td >

    <?php $sum2=0; foreach($contributions_ as $val){
        $sum = $sum2 + $contributions_[$val];
        }echo array_sum($contributions_);
      ?>

 </td>
</tr>

Here is the php:

<?php
 $tempArray = array(
  $rowRes['amount'],
  $rowRes['remarks'],
  $rowRes['ID']
 );               
 $contributions_[]  = $tempArray;
?>

I want to sum only amount(s) whiles the condition is true

This is my query which worked perfectly but cannot sum amount after displaying the various members contributions .

<?php $selectQuery = "select a.*,b.* ID, monthname(date) as month,month(date) as mnth, year(date) as yr, type,amount, recipient_initials,remarks, in_kind_items,other_type,other_payment_name,date from member_payments a inner join member b on a.memberID = b.ID";
            ?>

Why can't I sum amount here like SUM(amount)

You used $sum and $sum2 in this line:

 $sum = $sum2 + $contributions_[$val];

If I'm reading your code right, it should be more like this:

<?php $sum2=0; foreach($contributions_ as $val){
    $sum2 = $sum2 + $contributions_[$val];
    }echo $sum2;
?>

Sorry, I don't know what you mean by "I want to sum only amount(s) whiles the condition is true".

<tr >
     <td> Total </td>
     <td>
        <?php 
            $sum = 0; 
            foreach($contributions_ as $val){
                   $sum = $sum + $val['amount'];
            }

            echo $sum;
          ?>

     </td>
    </tr>

<?php
 $tempArray = array(
  $rowRes['amount'],
  $rowRes['remarks'],
  $rowRes['ID']
 );               
 $contributions_[]  = $tempArray;
 $sum += $rowRes['amount'];
?>

Got it now after echoing sum