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()?
You're misunderstanding how things work.
If you want to transfer certain variables from PHP to JavaScript, you could dump some output into JSON in your PHP script, like so:
$myVariable)));
/* Output looks like this:
[
{
"myVariable": "hello world"
}
]
*/
?>
Your JavaScript/JSON should look something like this:
$.getJSON("test.php", function(result) {
console.log(result[0].myVariable);
});
Does that make sense?