expected

<?php

if(isset($_POST['submit'])){
// REST Server URL
$request_url = 'http://localhost/drupal710/webservice/user/login';

// User data
$test =  array(
  'username' => $_POST['username'],
  'password' => $_POST['password'],
);
$user_data = json_encode($test);

// cURL
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $request_url);
curl_setopt($curl, CURLOPT_POST, 1); // Do a regular HTTP POST
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($user_data)); // Set POST data
curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);

$response = curl_exec($curl);

print $response;

$http_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
// Check if login was successful
if ($http_code == 200) {
  // Convert json response as array
  $logged_user = json_decode($response);
}
else {
  // Get error msg
  $http_message = curl_error($curl);
  die($http_message);
}

//print_r($logged_user);
}//endif
?>
<form action="" method="post">
User Name:<input type="text" name="username" maxlength="12" size="12"/> <br/>
Password:<input type="password" name="password" maxlength="36" size="12"/> <br/>
<p><input type="submit" name="submit" /></p>
</form>

original

<?php

if(isset($_POST['submit'])){
// REST Server URL
$request_url = 'http://localhost/drupal710/webservice/user/login';

// User data
$user_data = array(
  'username' => 'kankaung',
  'password' => '123456',
);

// cURL
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $request_url);
curl_setopt($curl, CURLOPT_POST, 1); // Do a regular HTTP POST
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($user_data)); // Set POST data
curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);

$response = curl_exec($curl);

print $response;

$http_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
// Check if login was successful
if ($http_code == 200) {
  // Convert json response as array
  $logged_user = json_decode($response);
}
else {
  // Get error msg
  $http_message = curl_error($curl);
  die($http_message);
}

//print_r($logged_user);
}//endif
?>
<form action="" method="post">
User Name:<input type="text" name="username" maxlength="12" size="12"/> <br/>
Password:<input type="password" name="password" maxlength="36" size="12"/> <br/>
<p><input type="submit" name="submit" /></p>
</form>