使用表单中的文本框设置值

Question

I have seen this code from https://www.codexworld.com to Create a URL Shortener using PHP.

// Include database configuration file
require_once 'dbConfig.php';

// Include URL Shortener library file
require_once 'Shortener.class.php';

// Initialize Shortener class and pass PDO object
$shortener = new Shortener($db);

// Long URL
$longURL = 'https://www.codexworld.com/tutorials/php/';

// Prefix of the short URL 
$shortURL_Prefix = 'https://localhost/'; // with URL rewrite
$shortURL_Prefix = 'https://localhost/?c='; // without URL rewrite

try{
    // Get short code of the URL
    $shortCode = $shortener->urlToShortCode($longURL);

    // Create short URL
    $shortURL = $shortURL_Prefix.$shortCode;

    // Display short URL
    echo 'Short URL: '.$shortURL;
}catch(Exception $e){
    // Display error
    echo $e->getMessage();
}

I have understood how the system goes but dont know how can this line of code $longURL = 'https://www.codexworld.com/tutorials/php/'; will not be based on static value but will be dynamic by collecting the link where the user will type it using a textbox input in a form?

How can I do that? Thank you in advance!