If I load a PHP page with Jquery .load(file.php), can the included file use the php variables that were defined on the page that called the load()?
variables scope in the PHP script loaded by JavaScript is different from the page that loaded the PHP script, so the answer is no.
PHP
JavaScript
but you can define global variables or use super global variables like ($_GET, $_POST, etc.) to get what you want.
$_GET
$_POST