Update a query by substracting a multiplicated number and using where - php

Question

I'm making a paid to click website and I'm looking to update a query by substracting and a multiplicated number.
I'm trying to get the users to buy some ads package, they enter the views count and the cost per view, then when they click on "Submit", it should update the user in the current session. However, it's not working: it's updating all the users balances.
Here's my code :
<?php
session_start();
require_once '../constants/initSite.php';
$siteConstant->addFile('css', 'account.css');
echo $siteConstant->getHead();
echo $siteConstant->getMenu();
if(isset($_SESSION['username']) && isset($_SESSION['password'])){
require_once '../constants/class.DatabaseConstants.php';
require_once '../class/class.DBase.php';
$done = false;
$db = new DatabaseConstants();
$dBase = new DBase($db->getHost(), $db->getUser(), $db->getPass());
$dBase->setDatabaseName($db->getDb());
if(!$dBase->connectDatabase()){
die('SQL ERROR at db class vd fn');
}
$userQuery = mysqli_query($dBase->getDbobj(), "SELECT * FROM members WHERE username=\"".$_SESSION['username']."\"");
if(mysqli_num_rows($userQuery)){
$userData = mysqli_fetch_assoc($userQuery);
}else{
die('User Not Found!');
}
if(isset($_POST['submit'])){
$adquery = 'INSERT INTO ads (Title,Link,ViewLimit,Pays) VALUES ("'.$_POST['title'].'","'.$_POST['link'].'","'.$_POST['views'].'","'.$_POST['pays'].'")';
mysqli_query($dBase->getDbobj(), $adquery);
if(!mysqli_affected_rows($dBase->getDbobj())<1){
$done = true;
}
$id = 0;
$qryE = mysqli_query($dBase->getDbobj(), 'SELECT Id FROM ads');
while($dataE = mysqli_fetch_assoc($qryE)){
$id = $dataE['Id'];
}
$userQueryE = mysqli_query($dBase->getDbobj(),'SELECT id FROM members');
while($userDataE = mysqli_fetch_assoc($userQueryE)){
mysqli_query($dBase->getDbobj(), 'INSERT INTO view (MemberId,AdId) VALUES ("'.$userDataE['id'].'","'.$id.'")');
}
$userquery = ('UPDATE members SET balance=("'.$userData['balance'].'" - "'.$_POST['views'].'" * "'.$_POST['pays'].'") WHERE username="'.$_SESSION['username'].'"') ;
$userData = mysqli_fetch_assoc($userQuery);
}
?>
<center><img style="margin-bottom:5px"src="../image/ad468x60.png"/></center>
<div id="maincontent" style="height:35em">
<center>
<ul id="menu2">
<li>Account Details</li>
<li>Add Advertisement</li>
<li>Request Payment</li>
<li>Referrals</li>
<li>Account Setting</li>
</ul>
<br>
</center>
<center><p style="color:#0481b1;"><b><u>Add Advertisements</u></b></p></center>
<form id="msform" method="POST" action="<?php echo $_SERVER["PHP_SELF"]; ?>">
<center>
<?php
if(isset($_POST['submit']) && !$done){
echo '<h3 style="color:red;">Something went wrong!</h3>';
}
if ($done){
echo '<h3 style="color:green;">Advertisement Added and is Active.</h3>';
}
?>
<?php
if($userData['balance'] < 5 ){
echo '<h3 style="color:red;">insufficient balance</h3></br>
<h2 style="color:red;text-decoration:underline">You currently have '.$userData['balance'].' '.$siteData['currency'].' </h2>
<center><h2 style="color:red">You do not have enough balance to add an advertissement.<br><br>You need to have at least 5 '.$siteData['currency'].' in your account balance to add an advertissement.</h2></center>';
}
else {
echo'
Title <input type="text" name="title" placeholder="Enter Site Title" required="true"/> <br><br>
Link <input type="text" name="link" placeholder="Enter Site Link" required="true"/><br><br>
Number of Viewers to send<input type="text" name="views" min="100" placeholder="Enter number of views to send(100 Views Minimum)" required="true"/><br><br>
Reward per view<input type="text" name="pays" min="0.00001" placeholder="Enter reward per view(0.00001 '.$siteData['currency'].' Minimum)" required="true"/><br><br>
<input type="submit" name="submit" class="NormButton" Value="Submit" /> ' ;
}
?>
</center>
</form>
</div>
<?php
require_once '../main/footer.php';
$dBase->closeDatabse();
}else{
header('location: index.php');
}
echo $siteConstant->getTail();

Answer 1

Why is your where clause is before SET
Check this
"UPDATE members SET balance=($userData['balance'] - $_POST['views'] * $_POST['pays']) WHERE username=$_SESSION['username']"