how to define variables as numeric

Hi, I have some issues with my code. It's to refresh account payfile at end of month. Comments are noted
in code (//).
#1) how to define variables as numeric(I've tried several examples(forums and manuals)
#2) how to add several variables together so they are known as numeric(they are all decimal 8,2 in the database)
#3) Please, someone explain the "Undefined variable: mysql".

the code:

<?php
//Open a new connection to the MySQL server
$link = mysqli_connect("localhost", "root", "", "prerentdb");

// Check connection
if($link === false){ die("ERROR: Could not connect. " . mysqli_connect_error()); }
//MySqli Select Query
$sql = "select * FROM payfile";
echo "<center>";echo date('m/d/y');echo "<br />";
$due=0;
  $prevbal="prevbal";
  $latechg="latechg";
  $secdep="secdep";
  $damage="damage";
$courtcost="courtcost";
  $nsf="nsf";
  $amtdue="amtdue";
  $amtpaid="amtpaid";
  $paidsum="paidsum";

$due = $prevbal + $latechg + $secdep + $damage + $courtcost + $nsf; // Warning: A non-numeric value encountered x 5 line 21
$amtdue = $amtdue + $due;  Warning: // A non-numeric value encountered x 1 line 22

// if no payment or partial payment, add $10 to latechg field and amount not paid to prevbal field
if ($amtpaid < $amtdue)  // Notice: Undefined variable: amtpaid
   { $latechg = $latechg + 10; $prevbal = $amtdue - $amtpaid;  }
// if payment = amtdue clear due
if ($amtpaid == $amtdue) // Notice: Undefined variable: amtpaid
   { $prevbal = 0; $latechg = 0; }
// if over-payment subtract over-payment
// from prevbal field
if ($ampaid > $amtdue )  // Notice: Undefined variable: amtpaid
{ $amtdue = $amtpaid  - $amtdue;  $prevbal = 0; $latechg = 0;  }
$secdep = 0; $damage = 0; $courtcost = 0; $nsf = 0;

// refresh every record - give every record the below values
$amtpaid = '0.00';
$hudpay = '0.00';
$datepaid = ' ';
$paidsum = '0.00';
$comments = ' ';

// Perform a query, check for error
if (!$mysqli -> query("UPDATE payfile SET // Undefined variable: mysqli
Fatal error: Uncaught Error: Call to a member function query() on null
prevbal='$prevbal',latechg='$latechg', hudpay='$hudpay', amtpaid='$amtpaid',
datepaid='$datepaid', comment='$comment', paidsum='$paidsum'
where unit = $unit"));
mysqli_query($sql) or die(mysql_error());
?>

PHP:

these are error messages:

Warning: A non-numeric value encountered in C:\xampp\htdocs\property\refreshpayments.php on line 21

Warning: A non-numeric value encountered in C:\xampp\htdocs\property\refreshpayments.php on line 21

Warning: A non-numeric value encountered in C:\xampp\htdocs\property\refreshpayments.php on line 21

Warning: A non-numeric value encountered in C:\xampp\htdocs\property\refreshpayments.php on line 21

Warning: A non-numeric value encountered in C:\xampp\htdocs\property\refreshpayments.php on line 21

Warning: A non-numeric value encountered in C:\xampp\htdocs\property\refreshpayments.php on line 21

Warning: A non-numeric value encountered in C:\xampp\htdocs\property\refreshpayments.php on line 22

Notice: Undefined variable: ampaid in C:\xampp\htdocs\property\refreshpayments.php on line 32

Notice: Undefined variable: mysqli in C:\xampp\htdocs\property\refreshpayments.php on line 44

Fatal error: Uncaught Error: Call to a member function query() on null on line 44

 

I'm not sure if I understood, "I'm wondering if
$prevbal="prevbal";
PHP:
is meant to be picking up a value from the SQL request... however you create the SQL for the request, you never actually go to the database to retrieve the data."

but I made these changes:
$sql = "select amtdue,amtpaid,paidsum,prevbal,latechg,secdep,damage,courtcost,nsf, FROM payfile";
$due=0;
$prevbal="prevbal";
$latechg="latechg";
$secdep="secdep";
$damage="damage";
$courtcost="courtcost";
$nsf="nsf";
$amtdue="amtdue";
var_dump(is_int($amtdue)); // this showed that it is not a number
otherwise, no change.

 

I so appreciate the advice. the numeric errors are gone but you see the comments below:

<?php
echo "<center>";echo date('m/d/y');echo "<br />";
//connect to database
$mysqli = new mysqli("localhost", "root", "", "prerentdb");

//check for errors
if ($mysqli->connect_errno) {
  echo "Failed to connect to MySQL: " . $mysqli->connect_error;
}

//MySqli Select Query
$res = $mysqli->query("SELECT  * FROM payfile");

//fetch query
$row = $res->fetch_assoc();
$due=0;
//display results
/* echo $row['amtdue'];
echo $row['amtpaid'];
echo $row['prevbal'];
echo $row['latechg'];
echo $row['secdep'];
echo $row['damage'];
echo $row['courtcost'];
echo $row['nsf'];
echo $row['paidsum'];
echo $row['comments']; */

if ($result = $mysqli->query($sql)) { // Undefined variable: sql& Warning:  Empty query
  while($obj = $result->fetch_object()){
  $due = $obj->prevbal + $obj->latechg + $obj->secdep + $obj->damage + $obj->courtcost + $obj->NSF;
  $amtdue = $obj->amtdue + due;
// do these need to be variables or can your code just use $obj?
  $prevbal =$obj->prevbal;
  $latechg = $obj->latechg;
  $secdep = $obj->secdep;
  $damage = $obj->damage;
  $courtcost = $obj->courtcost;
  $nsf = $obj->nsf;

// if no payment or partial payment, add $10 to latechg field and amount not paid to prevbal field
if ($amtpaid < $amtdue) 
   { $latechg = $latechg + 10; $prevbal = $amtdue - $amtpaid;  }
// if payment = amtdue clear due
if ($amtpaid == $amtdue)
   { $prevbal = 0; $latechg = 0; }
// if over-payment subtract over-payment
// from prevbal field
if ($ampaid > $amtdue ) 
{ $amtdue = $amtpaid  - $amtdue;  $prevbal = 0; $latechg = 0;  }
$secdep = 0; $damage = 0; $courtcost = 0; $nsf = 0;

// refresh every record - give every record the below values
$amtpaid = '0.00';
$hudpay = '0.00';
$datepaid = ' ';
$paidsum = '0.00';
$comments = ' ';

// Perform a query, check for error
if (!$mysqli -> query("UPDATE payfile SET 
prevbal='$prevbal',latechg='$latechg', hudpay='$hudpay', amtpaid='$amtpaid',
datepaid='$datepaid', comment='$comment', paidsum='$paidsum'
where unit = $ unit"));
mysqli_query($sql) or die(mysql_error());
  // now you have to do something with them before the loop ends and it gets a new record
  }
  }
  $result->close(); // Fatal error:  Call to a member function close() on bool
?>

PHP:

 

I 'm trying to recode to prepared and got kind of balled up. My one error displayed is commented ( //)
in the code on line 18. Am I getting closer?

<?php
echo "<center>";echo date('m/d/y');echo "<br />";
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "prerentdb";

// Create connection
$mysqli = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($mysqli->connect_errno) {
  echo "Failed to connect to MySQL: " . $mysqli->connect_error;
  exit();
}

$sql = "SELECT * FROM payfile WHERE id=?";
$stmt = prepared_query($conn, $sql, [$id]); // *** line 18 Fatal error: Uncaught Error: Call to undefined function prepared_query() ***
$payfile = $stmt->get_result()->fetch_all(MYSQLI_ASSOC);
$due=0;

   while($obj = fetch_object()){
  $due = $obj->prevbal + $obj->latechg + $obj->secdep + $obj->damage + $obj->courtcost + $obj->NSF;
  $amtdue = $obj->amtdue + due;

// if no payment or partial payment, add $10 to latechg field and amount not paid to prevbal field
if ($amtpaid < $amtdue) 
   { $latechg = $latechg + 10; $prevbal = $amtdue - $amtpaid;  }
// if payment = amtdue clear due
if ($amtpaid == $amtdue)
   { $prevbal = 0; $latechg = 0; }
// if over-payment subtract over-payment
// from prevbal field
if ($ampaid > $amtdue ) 
{ $amtdue = $amtpaid  - $amtdue;  $prevbal = 0; $latechg = 0;  }
$secdep = 0; $damage = 0; $courtcost = 0; $nsf = 0;

// refresh every record - give every record the below values
$amtpaid = '0.00';
$hudpay = '0.00';
$datepaid = ' ';
$paidsum = '0.00';
$comments = ' ';

// prepare and bind
$stmt = $mysqli->prepare("UPDATE payfile SET prevbal=?, latechg=?, hudpay=?, amtpaid=?, datepaid=?,
comment=?, paidsum=? WHERE unit=?");

$stmt->bind_param("iiiissi", $prevbal, $latechg, $hudpay, $amtpaid, $datepaid, $comment, $paidsum);

// set parameters and execute
$prevbal = "prevbal";
$latechg = "latechg";
$hudpay = "hudpay";
$amtpaid = "amtpaid";
$datepaid = "datepaid";
$comment = "comment";
$paidsum = "paidsum";
$unit = "unit";

if ($stmt->execute()) {
  echo "updated successfully";
} else {
  echo "Failed to update Item";
}

//var_dump($stmt);

$stmt->close();
$mysqli->close();
   }
  ?>

PHP:

 

In trying to resolve previous issues and further researching, new code produced new messages.
The instruction for preparing the "select" obviously were not right?

<?php
echo "<center>";echo date('m/d/y');echo "<br />";
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "prerentdb";
// Create connection
$mysqli = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($mysqli->connect_errno) {
  echo "Failed to connect to MySQL: " . $mysqli->connect_error;
  exit();
}

// Select some data with unit as criteria
  $statement = $mysqli->prepare("select unit, $amtpaid, $datepaid, $prevbal, $latechg, $hudpay,  $comment, $paidsum from payfile where unit = ?");

  $statement->bind_param("s", $unit);
  // Fill our parameters
  $unit = $_POST["unit"];
    $amtpaid = $_POST["amtpaid"];
  $datepaid = $_POST["datepaid"];
  $prevbal = $_POST["prevbal"];
    $latechg = $_POST["latechg"];
  $hudpay = $_POST["hudpay"];
    $comment = $_POST["comment"];
  $paidsum = $_POST["paidsum"];
      
  $statement->execute(); // Execute the statement.
  $result = $statement->get_result(); // Binds the last executed statement as a result.
$stmt = $mysqli->prepare($sql);
$stmt->bind_param($unit); //Warning: Wrong parameter count for mysqli_stmt::bind_param() line 19 

$payfile = $stmt->get_result()->fetch_all(); //Fatal error: Uncaught Error: Call to a member function fetch_all() on bool line 21
$due=0;
while($obj = fetch_object()){
  $due = $obj->prevbal + $obj->latechg + $obj->secdep + $obj->damage + $obj->courtcost + $obj->NSF;
  $amtdue = $obj->amtdue + due;
// if no payment or partial payment, add $10 to latechg field and amount not paid to prevbal field
  $latechg = 0;
  $prevbal = 0;
  if ($obj->amtpaid < $obj->amtdue) {
  $latechg += 10;
  $prevbal = $obj->amtdue - $obj->amtpaid;
}
// if over-payment subtract over-payment
// from prevbal field
if ($amtpaid > $amtdue ) {
  $amtdue = $amtpaid  - $amtdue;
}
// $nsf doesn't get used or saved
$secdep = $damage = $courtcost = $nsf = 0;
// refresh every record - give every record the below values
$amtpaid = '0.00';
$hudpay = '0.00';
$datepaid = ' ';
$comment = ' ';
$paidsum = '0.00';
// prepare and bind
$stmt = $mysqli->prepare("UPDATE payfile SET prevbal=?, latechg=?, hudpay=?, amtpaid=?, datepaid=?,
  comment=?, paidsum=? WHERE unit=?");
$stmt->bind_param($prevbal, $latechg, $hudpay, $amtpaid, $datepaid, $comment, $paidsum, $unit);
if ($stmt->execute()) {
  echo "updated successfully";
} else {
  echo "Failed to update Item";
}
//var_dump($stmt);
$stmt->close();
}
$mysqli->close();
?>

PHP:

result:

Notice: Undefined variable: amtpaid, datepaid, prevbal, latechg, hudpay, comment, paidsum on line 16
How to define here?

Fatal error: Uncaught Error: Call to a member function fetch_all() on bool line 18
how to resolve?

 

I took out the statement with "id" only get the fatal error and don't
know how to resolve?
<?php
echo "<center>";echo date('m/d/y');echo "<br />";
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "prerentdb";
// Create connection
$mysqli = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($mysqli->connect_errno) {
echo "Failed to connect to MySQL: " . $mysqli->connect_error;
exit();
}

$unit = filter_input(INPUT_GET, 'unit');
$sql = "SELECT * FROM payfile WHERE unit=?";

$stmt = $mysqli->prepare($sql);
// $stmt->bind_param($id);
$payfile = $stmt->get_result()->fetch_all();
$due=0;
while($obj = fetch_object()){
// is this add up right?
$due = $obj->prevbal + $obj->latechg + $obj->secdep + $obj->damage + $obj->courtcost + $obj->NSF;
$amtdue = $obj->amtdue + due;
// if no payment or partial payment, add $10 to latechg field and amount not paid to prevbal field
$latechg = 0;
$prevbal = 0;
if ($obj->amtpaid < $obj->amtdue) {
$latechg += 10;
$prevbal = $obj->amtdue - $obj->amtpaid;
}
// if over-payment subtract over-payment
// from prevbal field
if ($amtpaid > $amtdue ) {
$amtdue = $amtpaid - $amtdue;
}
// $nsf doesn't get used or saved
$secdep = $damage = $courtcost = $nsf = 0;
// refresh every record - give every record the below values
$amtpaid = '0.00';
$hudpay = '0.00';
$datepaid = ' ';
$comment = ' ';
$paidsum = '0.00';
// prepare and bind
$stmt = $mysqli->prepare("UPDATE payfile SET prevbal=?, latechg=?, hudpay=?, amtpaid=?, datepaid=?,
comment=?, paidsum=? WHERE unit=?");
$stmt->bind_param($prevbal, $latechg, $hudpay, $amtpaid, $datepaid, $comment, $paidsum, $unit);
if ($stmt->execute()) {
echo "updated successfully";
} else {
echo "Failed to update Item";
}
//var_dump($stmt);
$stmt->close();
}
$mysqli->close();
?>

 

am i not