WP从AJAX中的另一个函数返回值

Question

I have the following JS script:

var data = {
    action: 'my_action',
    sku: SKU
};
$.ajax({
    type:"POST",
    url:myObject.url,
    data: data,
    dataType: 'json',
    success: function (response) {
        console.log(response);
    },
    error: function(jqXHR, textStatus, errorThrown) 
    {
      alert(errorThrown);
    }
});

Here I am calling a PHP function my_action. In PHP I construct the function and the callback properly, and have these two functions:

function test_function()
{
    return "Test";
}

function my_action_callback() {
    $response = test_function();
    echo json_encode($response);
    die();
}

This causes an error inside the alert: "Unexpected end of input".

However, should the callback look like this:

function my_action_callback() {
    $response = "Test";
    echo json_encode($response);
    die();
}

I get "Test" returned in JS.

Why doesn't the first snippet work?

Edit

In fact, the error seems to stem from me calling the my_test_function in PHP. The following PHP code also returns the unexpected end of input error:

function test_function()
{
    return "Test";
}

function my_action_callback() {
    $response = test_function();
    $response = "Test";
    echo json_encode($response);
    die();
}