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$dbh = new PDO("MySQL:host=$hostname;dbname=$db", $username, $password);
foreach($dbh->query('SELECT SUM(total_cost) 
FROM purchase') as $row) {
echo "<tr>";
echo "<td>" . $row['SUM(total_cost)'] . "</td>";
echo ""; 
}

$sql__total__seventh = "SELECT SUM(total) FROM $whichTable2";

echo "<tr>\n";
echo "<td>TOTALS</td>";
foreach($conn->query($sql__total__seventh) as $row__total__seventh) {
	
	$total__seventh = $row__total__seventh['SUM(total)'];
		
	echo "<td>$total__seventh</td>";
}