Why does JS fetch retrieve php code instead of php output?

Question

I'm new with php and I'm trying to retrieve some data from a database in json encoding and then I want to retrieve the output with javascript. So I defined the following php functions in functions.php:

<?php
function getConn(){
  $USER='root';
  $PASSWORD='mypass';
  $HOST='localhost';
  $DATABASE='dbname';
  try {
    $connection = new PDO("mysql:host=$HOST;dbname=$DATABASE", $USER, $PASSWORD);
    return $connection;
  } catch (PDOException $e) {
    exit("Error: " . $e->getMessage());
  }
}
    

function getData($connection){
  ini_set('display_errors', 1);
  ini_set('display_startup_errors', 1);
  error_reporting(E_ALL);
  $connection->query("SET NAMES 'UTF8'");
  $query = $connection->query("SELECT * FROM table;");
  $results = $query->execute();
  if ($results) {
    $jobs = $query->fetchALL(PDO::FETCH_ASSOC);
    header('Content-Type: application/json');
    return json_encode($jobs, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES);
  } else{
    echo "ERROR <br>";
    print_r($connection -> errorInfo());
  }        
}
?>

At this point, I have my main.php, wich simply does:

<?php
  ini_set('display_errors', 1);
  ini_set('display_startup_errors', 1);
  error_reporting(E_ALL);  
  include('functions.php');
  $connection = getConn();
  echo getData($connection);
?>

If I go to the page http://localhost:5500/main.php I can see the correct data, in correct Json format.

Client-side, I have an html page with the following javascript:

"use strict";
async function getData(){
  let myHeaders = new Headers();
  myHeaders.append("Content-Type", "application/json; charset=UTF-8");

  let res = await fetch("http://localhost:5500/main.php", myHeaders);
  let data = await res.json(); // changed in: let data = await res.text();
  console.log(data);
}
getData();

This code made error: Uncaught (in promise) SyntaxError: Unexpected token < in JSON at position 0. So, I tried to log the response text and what is printed is the main.php code.

Why does it happen? I expected the json to be retrieved which is the php output, instead of the php code.
Thanks in advance.

EDIT: since the question is marked as duplicated, I want to be clearer: the suggested answer is this, but my situation is different: my php code is executed. In fact, visiting the web page via chrome or firefox, I can display the php output as page text. What I can't do is to retrieve this output via javascript.