Javascript Fetch With JSON Body to PHP Then Back Again

Question

I have being trying to search this here on Stackoverflow, but because I do not know what it is called I am having trouble.

How do I do this?:

My Javascript code looks like this:

return fetch('http://www.mywebsite.com/backend.php', {
    method: "post",
    headers: {
        'content-type': 'application/json'
    },
    body: JSON.stringify({
        myFloat: 3.14,
        myString: "Hello"
    })
})
.then((response) => response.json())
.then((data) => {
    // How do I get these?
    let returnFloat = ???;
    let returnString = ???;
});

So what does my php file look like?

<?php
    $myFloat = ?????;
    $myString = ?????;

    $returnFloat = 2.0 * $myFloat;
    $returnString = $myString . " and Goodbye"; // "Hello and Goodbye"

    // Now how do I pack this up to send it back???
    // I want to send back $returnFloat and $returnString
?>

So my questions are these:

1. How do I set $myFloat = ???; and $myString = ???; making sure these are a number and a string respectively (and not both strings)?

2. In the php I have the line // Now how do I pack this up to send it back???. I want to send back $returnFloat and $returnString. How do I pack them up to send back?

3. Once these two bits of php data in (2) are sent back, how do I unpack it back in my javascript with the lines let returnFloat = ???; and let returnString = ???; making sure both are a float and a string respectively?

4. Are there any issues of cross-website security I need to know about? Ie htaccess or the windows server equivalent. If there are, what do they look like?