I am getting unwanted null with my json output in json-api wordpress

I am new in wordpress and using Json-Api in wordpress for web-services, and i have create my own service in json-api/controllers/core.php file my function is:

<?php
public function get_blogs(){
global $json_api;
$categories = $json_api->introspector->get_categories($args);
echo  "[";
foreach($categories as $cat){
if($cat->id == "5432"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5433"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5439"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5444"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5440"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5438"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5430"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
 if($cat->id == "5435"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5436"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5434"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5441"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5445"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5431"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5437"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "203"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}".",";
}
if($cat->id == "5442"){
 $catPost = get_posts('cat="$cat->id"&posts_per_page=4');
 echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
  $catnow = json_encode($catPost);
  echo $catnow;
  echo "}".",";
  }
  if($cat->id == "5447"){
  $catPost = get_posts('cat="$cat->id"&posts_per_page=4');
  echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
  $catnow = json_encode($catPost);
  echo $catnow;
  echo "}".",";
  }
  if($cat->id == "530"){
  $catPost = get_posts('cat="$cat->id"&posts_per_page=4');
  echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
  $catnow = json_encode($catPost);
  echo $catnow;
   echo "}".",";
  }
  if($cat->id == "5443"){
  $catPost = get_posts('cat="$cat->id"&posts_per_page=4');
  echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
  $catnow = json_encode($catPost,JSON_PRETTY_PRINT);
  echo $catnow;
  echo "}".",";
  }
if($cat->id == "5448"){
$catPost = get_posts('cat="$cat->id"&posts_per_page=4');
echo "{"."\"category_title\":\"$cat->title\"".",\"category_id\":\"$cat->id\"".",\"posts\":";
$catnow = json_encode($catPost);
echo $catnow;
echo "}";
}

}
echo  "]";
  }

after that when i am getting its output as i want ,one null is extra added in json after finish my json output "]null" in last, please help.