demo.php

<?php
    function fetchData()
    {
        $inputText = file_get_contents("php://input");
        $userInputs = explode("&", $inputText);

        $data = array();

        foreach ($userInputs as $input) {
            $keyValue = explode("=", $input);
            $key = $keyValue[0];
            $value = $keyValue[1];

            $data[$key] = $value;
        }

        return $data;
    }

    $data = fetchData();

    $username = $data['username'];
    $password = $data['password'];

    header('Content-Type: application/json');
    echo json_encode($data);
 ?>