Warning: use of undefined constant

Hi, I get the following when I try to execute:
What is wrong?

expiry was updated successfully.
Warning: Use of undefined constant expiry - assumed 'expiry' (this will throw an Error in a future version of PHP) on line 14

Warning: Use of undefined constant expiry - assumed 'expiry' (this will throw an Error in a future version of PHP) on line 15

Warning: Use of undefined constant expiry - assumed 'expiry' (this will throw an Error in a future version of PHP) on line 16
---------------------------------------------------------------
the code
<?php
$link = mysqli_connect("localhost", "root", "", "prerentdb"); 
// Check connection
if($link === false){ die("ERROR: Could not connect. " . mysqli_connect_error()); }
echo "<center>";echo date('m/d/y');
echo "<br />";
$id='id';
$expiry='expiry';

$sql = "UPDATE paytbl SET $expiry = $expiry-1 where id = '$id'";
if(mysqli_query($link, $sql)){ echo "expiry was updated successfully."; } 
else { echo "ERROR: Could not able to execute $sql. " . mysqli_error($link); }

if(expiry == 7) {echo "1 week remaining on contract";} 
elseif (expiry == 1) {echo "1 day remaining on contract";}
elseif (expiry == 0) {echo "contact xxxxx.com to renew and continue use";}

// Close connection
mysqli_close($link);
?>

the code
<?php
$link = mysqli_connect("localhost", "root", "", "prerentdb"); 
// Check connection
if($link === false){ die("ERROR: Could not connect. " . mysqli_connect_error()); }
echo "<center>";echo date('m/d/y');
echo "<br />";
$id='id';
$expiry='expiry';

$sql = "UPDATE paytbl SET $expiry = $expiry-1 where id = '$id'";
if(mysqli_query($link, $sql)){ echo "expiry was updated successfully."; } 
else { echo "ERROR: Could not able to execute $sql. " . mysqli_error($link); }

if(expiry == 7) {echo "1 week remaining on contract";} 
elseif (expiry == 1) {echo "1 day remaining on contract";}
elseif (expiry == 0) {echo "contact xxxxx.com to renew and continue use";}

// Close connection
mysqli_close($link);
?>