problema con timestamp

[ufficio]

ho fatto così ma mi da questo errore:
Fatal error: Call to undefined function: where() in /web/htdocs/www.sito.it/home/include_login_serviziounghie.php on line 90

<?php
$timestamp = time();
$timeout = 300;

where (timestamp + $timeout) < UNIX_TIMESTAMP();


$inserisci = "INSERT INTO useronline (timestamp, id_login, ip, file)
VALUES ('$timestamp','$_SESSION[idlogin]','$REMOTE_ADDR','$PHP_SELF')";
mysql_query($inserisci, $conn) or die (mysql_error() );

if(!($inserisci)) { 
print "Useronline Insert Failed > "; 
}

$cancella = "DELETE FROM useronline WHERE timestamp<$timeout";
mysql_query($cancella, $conn) or die (mysql_error() );

if(!($cancella)) { 
print "Useronline Delete Failed > "; 
} 


?>