If you dont want to use a database then you could use a text file however it may be more difficult.

Are you after a single password for an area that anyone can use? Or do you want to be able to give individuals there own password then stop its use once they have used it?

How foolproof do you want your system? If it is only to stop average people then use a cookie to say that they have already accessed the page and not allow them access again. It wont stop those that know what they are doing but would be a lot easier.

If you are just going to pick a list of passwords that anyone can use, then you dont need an external file. Just put the list in your php code i.e.

$avail_passwords = array('letmein', 'pass2', 'pass3', 'etc');
if (in_array [uk3.php.net]($_POST['password'], $avail_passwords) {
// let them in
}
else {
// dont let them in
}

You can also store an array of username/password combinations in the php script directly.

$valid = array(
'jane' => 'word',
'joe' => 'pass'
);
if( isset($valid[$_POST['username']]) && $_POST['password'] == $valid[$_POST['username']]){
// let them in
} else {
// dont let them in
}

OP wants one time passwords. So password has to be deleted after used once. So extending script :)

------------- login.php ---------------
include('passwords.php');

if( isset($valid[$_POST['username']]) && $_POST['password'] == $valid[$_POST['username']]){
// let them in

// remove password from array
unset($valid[$_POST['username']]);

// save new list to file
$f = fopen('passwords.php','w+');
fputs($f, '<'.'?'."php\n");
fputs($f, '$valid = array('."\n");
foreach($valid as $u => $p) {
if (isset($valid[$u]))
fputs($f, "'$u' => '$p'\n");
}
fputs($f, ');');
fputs($f, '?'.'>');
fclose($f);

} else {
// dont let them in
}

--------- passwords.php -----------------
$valid = array(
'jane' => 'word',
'joe' => 'pass'
);