$amtpaid = is decimal 530.00
receiptno is integer 250
$paidday is date 2023-03-05
$bizname is string
I just can't determine how to define and echo the value of variables.

<!DOCTYPE html><html>
<head>
     <title>record payment</title>
<html><head>
<script type="text/javascript">
var monthNames = [ "January","February","March","April","May","June","July",
                   "August","September","October","November","December"     ];
var today = new Date();  
var date = monthNames[today.getMonth()] + " - " + today.getDate() + " - " + today.getFullYear();
</script>
</head>
<body><center>
Date:<script type="text/javascript">document.write(date);</script><p>
<img src="apt-pic.jpg" alt="apartment" height=100 width=300><p>
<?php
$link = mysqli_connect("localhost", "root", "", "homedb"); 
// Check connection
if($link === false){ die("ERROR: Could not connect. " . mysqli_connect_error()); }

$unit=$_POST['unit'];
$amtpaid=$_POST['amtpaid'];
$paidday=$_POST['paidday'];
$id = '';

$amtpaid = 0;
$amtdue =  0;
$dueday = '';

$prevbal =  0;
$latechg =  0;

$secdep =  0;
$damage =  0;
$courtcost =  0;
$nsf =  0;
$paidday = ''; 

$bizname = '';
$bizstreet = '';
$bizcity = '';
$bizzip = '';
$bizemail = '';
$receiptno = 0;
$owed = $amtdue - $amtpaid;
$due= '';
/* Perform a query, check for error */
$sql = "UPDATE crttbl SET 
receiptno = receiptno + 1 where id=1";

echo "receipt# $receiptno";  // line 50

 if(mysqli_query($link, $sql)){ echo "receiptno updated"; } 
else { echo "ERROR: Could not able to execute $sql. " . mysqli_error($link); }

$due = $amtdue + $prevbal + $latechg + $secdep + $damage + $courtcost + $nsf;

  /* if no pay or part pay, add $35 to latechg field and amount not paid to prevbal field */
    if ($amtpaid < $due) { $prevbal = $due - $amtpaid; $latechg = $latechg + 35.00; 
$amtpaid = 0; $secdep = 0; $damage = 0;  $courtcost = 0;
$nsf = 0; $hudpay = 0; $comments = ' '; $paidday = ' '; }
/* if amt paid = amt due,  */
    if ($amtpaid == $due) { $amtpaid = 0; $prevbal =  0; $latechg = 0;; 
 $secdep = 0; $damage = 0;  $courtcost = 0;
$nsf = 0; $hudpay = 0; $comments = ' '; $paidday = ' '; }  
/* if over-payment subtract over-payment from prevbal */
    if ($amtpaid > $due)  { $prevbal = $amtpaid - $due;
$amtpaid = 0; $latechg = 0; $secdep = 0; $damage = 0;  $courtcost = 0;
$nsf = 0; $hudpay = 0; $comments = ' '; $paidday = ' '; } 

/* Perform a query, check for error */
$sql = "UPDATE paytbl SET 
$amtpaid = '$amtpaid', dueday = DATE_ADD(dueday, INTERVAL 1 MONTH), prevbal = '$prevbal', 
latechg = '$latechg', paidday = '$paidday' WHERE unit = '$unit'";
 if(mysqli_query($link, $sql)){ echo ""; } 
else { echo "ERROR: Could not able to execute $sql. " . mysqli_error($link); }
?>

<?php echo $bizname;echo "<br />"; ?>  // line  78 
<?php echo $bizstreet;echo "<br />"; ?>  
<?php echo $bizcity;echo "<br />"; ?>  
<?php echo $bizzip;echo "<br />"; ?><br>  
<?php echo $bizemail;echo "<br />"; ?>  // line 82  

For:<SELECT name="options">
<option value="#990033" style="background-color: Violet;">Rent payment</option>
<option value="#003300" style="background-color: Aquamarine;">Background Check</option>
<option value="#6600cc" style="background-color: Pink;">Security Deposit Payment</option>
<option value="#003300" style="background-color: Aquamarine;">Damages Payment</option>
<option value="#990033" style="background-color: Violet;">Late Charges Payment</option>
<option value="#003300" style="background-color: Aquamarine;">Court Costs Payment</option>
<option value="#6600cc" style="background-color: Pink;">NSF Payment</option>
<option value="#990033" style="background-color: Violet;"> </option>
</SELECT><p>

Tenant paying is: <?php echo $_POST["unit"]; ?> - 
Amount paid is: <?php echo $_POST["amtpaid"]; ?> - 

Balance due is:<?php echo $owed; ?><br>  // line 98 
<b><input type="text" size="35" maxlength="35" name=frm.Name" value="sign" STYLE="color: #000000; font-weight: bold; background-color: #ffccff;" onFocus="this.value=''"><br>
<h3>We Thank You</h3>
</center></body></html>
<?php
 // Close connection
mysqli_close($link);
?>

result:
 apartment
receipt# 0 receiptno updatedERROR: Could not able to execute UPDATE paytbl SET 0 = '0', 
dueday = DATE_ADD(dueday, INTERVAL 1 MONTH), prevbal = '0', latechg = '0', paidday = ' ' 
WHERE unit = 'unit1'. You have an error in your SQL syntax; check the manual that corresponds 
to your MariaDB server version for the right syntax to use near '0 = '0', dueday = DATE_ADD(dueday, 
INTERVAL 1 MONTH), prevbal = '0', latech...' at line 2
// line 78 

// line 82 For: Tenant paying is: unit1 - Amount paid is: 530.00 - Balance due is:0
// line 98
$sql = "UPDATE paytbl SET 
amtpaid = '$amtpaid', dueday = DATE_ADD(dueday, INTERVAL 1 MONTH), prevbal = '$prevbal', 
latechg = '$latechg', paidday = '$paidday' WHERE unit = '$unit'";
 if(mysqli_query($link, $sql)){ echo ""; } 
else { echo "ERROR: Could not able to execute $sql. " . mysqli_error($link); }

I think $amtpaid = '$amtpaid' should be: amtpaid = '$amtpaid'

THANKS SO MUCH. Only prob now is the receiptno and owed echo as 0?

See the flaw in the following:

$amtpaid=$_POST['amtpaid'];
$paidday=$_POST['paidday'];

//...

$amtpaid = 0;
$amtdue =  0;

// ...

$receiptno = 0;
$owed = $amtdue - $amtpaid;
this is what is displayed:
receipt# 0
Tenant paying is: unit1 - Amount paid is: 530.00 - Balance due is:-530

the receiptno is 250
the amtdue is 530.00, amtpaid is 530.00, balance due should be 0

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