PHP使用SELECT SUM

I am new to PHP, and was wondering if you could help with my select sum query?

I am wanting to add the total amount from my expenses table in my database based on user_id?

Here is what I have so far

<?php
    //sets up thisPage          
    $pageSize=10;
    if (isset($_POST["thisPage"])) 
        $thisPage = $_POST["thisPage"];
    else 
        $thisPage=1;

    //selects all distinct expenses that have been uploaded
    $dbQuery="SELECT SUM(amount) AS TotalExpenditure FROM expenses WHERE user_id =       '$userID'; ";

    echo $dbQuery;

    $dbResult=mysqli_query($db_connection, $dbQuery) or die (mysqli_error($db_connection));

    echo "<table> <thead>";
    //echo '<tr> <th>Project ID</th><th>Project Name</th></tr> </thead>';
    while ($dbRow=mysqli_fetch_array($dbResult)){
        // display row with expense
        echo '<tr> <td>'. $dbRow['amount'] .'</td>';
    }
    echo "</table>";
    echo "</form>";

?>

You have

$dbRow['amount']

but you use

SUM(amount) AS TotalExpenditure
             // ^ this is the name (alias) of your column now

TotalExpenditure is the name of the amount, so the last part should be

echo '<tr> <td>'. $dbRow['TotalExpenditure'] .'</td> </tr>';

You have to use TotalExpenditure as key of your array, so

$dbRow['TotalExpenditure'] instead of $dbRow['TotalExpenditure']?

You're giving an alias to your count() sql function so you have to use it