updating names not values


I'm updating a database table with an HTML form. The update occurs but with the field names
not the values?
the form code:

<!DOCTYPE html><html lang="en">
<body bgcolor="#ccffff"><center>
<font size=+2><b> update 515 Certification Expiration database</font><br>
<form action="cert-update.php" method="post">
<label for="unit">Unit:</label>
<input type="text" name="unit" id="unit">
<label for="tenant">Tenant:</label>
<input type="text" name="tenant" id="tenant">
<label for="effdate">Effective date:</label>
<input type="text" name="effdate" id="effdate">
<label for="expdate ">Expiration date :</label>
<input type="text" name="expdate " id="expdate "><br>

<label for="moveindate">Move-in date:</label>
<input type="text" name="moveindate" id="moveindate">
<label for="daysleft">Days left:</label>
<input type="text" name="daysleft" id="daysleft">
<label for="date90">date90:</label>
<input type="text" name="date90" id="date90">
<label for="date60">date60:</label>
<input type="text" name="date60" id="date60">
<label for="date30">date30:</label>
<input type="text" name="date30" id="date30"><br>

<input type="submit" name="submit" value="Submit">
the update code:

<center><b><font size=+2> 515 Certification Expiration Update</font><br>06/22/2020 - 06/22/2021<p>

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "prerentdb";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);


$sql = "UPDATE certtbl SET
unit = '$unit',
tenant = '$tenant',
effectdate = '$effectdate',
expdate = '$expdate',
moveindate = '$moveindate',
daysleft = '$daysleft',
date90 = '$date90',
date60 = '$date60',
date30 = '$date30'
WHERE unit='$unit'";
if ($conn->query($sql) === TRUE) { echo "Record updated successfully"; }
else { echo "Error updating record: " . $conn->error; }