Repeat PHP function every 2 seconds

So I have a code, where a number is retrieved from a text file and is displayed through HTML.

The thing is that it only retrieves the number when I refresh the page. If I change the number on other page, it doesn't change on the other page.

I've seen similar questions before, most of them said to use AJAX, but I don't understand much about AJAX and I only need it for this bit of code, it's not like I'm going to constantly use it. I would like to know if there is any other besides AJAX and if there is only AJAX, please present examples of code.

PHP code:

<?php
    $file = "num.txt"; //Path to your *.txt file
    $contents = file($file);
    $num = implode($contents);
?>

JavaScript code. Basically it gets the PHP value and outputs to the html.

document.getElementById("num").innerHTML = '<?php echo $num; ?>';

Keep in mind, I don't want to refresh the page. Just the variable.

EDIT: PHP code - Getting an error - Notice: Undefined index: keynum in C:\xampp\htdocs\num.php on line 3

Here is the code of the PHP

<?php
    $keynum = $_POST['keynum'];
    $post = "INSERT INTO post (keynum) VALUES ($keynum)";
    $fileLocation = getenv("DOCUMENT_ROOT") . "/num.txt";
    $file = fopen($fileLocation,"w");
    $content = $keynum;
    fwrite($file,$content);
    fclose($file);
    echo 'Response';
    die();
 ?>

Solution

get_nbr.php

 <?php
 $file = "num.txt"; //Path to your *.txt file
 $contents = file($file);
 $num = implode($contents);
 echo $num;
 ?>

index.php

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<!---trigger get_nbr() on page load on an infinite loop every two seconds -->
<body onload="setInterval(get_nbr, 2000)">
<script>
  function get_nbr() {
  var xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
  document.getElementById("num").innerHTML = this.responseText;
  }
  };
  xhttp.open("GET", "get_nbr.php", true);
  xhttp.send();
  }
 </script> 
<p id="num"></p>