Noob question -- Dynamic text?: index.php?keyword=test

So I want to have dynamic text depending on the url. For example if the url is:

www.mysite.com/index.php?variable=test

I then want to have a page with a paragraph of text that has a variable. So everytime the website has $variable in the text, it replaces the $variable with test or whatever I specify in the URL. How do I do this and what is it called? I tried searching google for dynamic php text but I didnt find anything on how to actually do it. Thanks,

 

<?php
// www.mysite.com/index.php?variable=test

if (array_key_exists('variable', $_GET)) {
    $variable = $_GET['variable'];
}

?>

<!-- your paragraph -->
blah blah blah blah <?php echo $variable; ?>

or

blah blah blah blah <?=$variable?>

PHP:

 

Or you could do something like (1 line code):

<?
if (isset($_GET['variable']) echo $_GET['variable'];  // You may want to sanitize this var as it can lead to security holes.
?>

PHP: