Generating a Non-Repeating Random Number - PHP

I'm trying to get a custom function in php to return a random number between 1 and 20 that does not repeat i.e. produce the same number more than once, since I need to subsequently use this number to navigate to one of twenty web pages, and I don't want the same web page displayed.

Here is my code in three steps:

<form action="rand.php">
        <p>Click this button to display a random number that does not repeat...</p>
        <p><input type="submit" value="Generate"></p>
    </form>

Here is rand.php:

require_once('functions.php');

$page = generateNumber();
echo $page;

Here is functions.php:

<?php

$check = array();

function generateNumber() {

    global $check;

    $page_no = mt_rand(1,20);
    $check[] = $page_no;

    if (count($check) != 1) {

        foreach ($check as $val) {
            if ($val == $page_no) {
                $page_no = mt_rand(1,10);
                continue;
            }
        }
        return $page_no;
    }
    else {
        return $page_no;
    }
}

?>

My code seem to be functioning, however, it is repeating numbers so I am obviously doing something wrong. The reason I initially check the count is so that is returns the first number regardless, since it would be a single fresh number.

In order to see the number change I have been refreshing the rand.php page in my browser.