<?
//Authentication rest API magento2.Please change url accordingly your url

//I HAVE CHANGED EXAMPLE.COM,USERNAME AND PASSWORD FOR SECURITY PURPOSES

$adminUrl='https://example.com/index.php/rest/V1/integration/admin/token';
$ch = curl_init();
$data = array("username" => "THEUSERNAME", "password" => "THEPASSWORD");
$data_string = json_encode($data);
$ch = curl_init($adminUrl);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
        'Content-Type=> application/json',
        'Content-Length=> ' . strlen($data_string))
);
$token = curl_exec($ch);
$token=  json_decode($token);

//Use above token into header
$headers = array('Authorization=> Bearer $token');

$requestUrl='https://example.com/index.php/rest/V1/products/{MY_SKU}/stockItems/827';
//Please note 24-MB01 is sku

$ch = curl_init();
$ch = curl_init($requestUrl);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
$post ='{
  "stockItem": {
    "item_id": 12,
    "product_id": 0,
    "stock_id": 0,
    "qty": 4,
    "is_in_stock": true,
    "is_qty_decimal": true,
    "show_default_notification_message": true,
    "use_config_min_qty": true,
    "min_qty": 0,
    "use_config_min_sale_qty": 0,
    "min_sale_qty": 0,
    "use_config_max_sale_qty": true,
    "max_sale_qty": 0,
    "use_config_backorders": true,
    "backorders": 0,
    "use_config_notify_stock_qty": true,
    "notify_stock_qty": 0,
    "use_config_qty_increments": true,
    "qty_increments": 0,
    "use_config_enable_qty_inc": true,
    "enable_qty_increments": true,
    "use_config_manage_stock": true,
    "manage_stock": true,
    "low_stock_date": "string",
    "is_decimal_divided": true,
    "stock_status_changed_auto": 0,
    "extension_attributes": {}
  }
}';

$options = array(
    CURLOPT_URL=>$toURL,
    CURLOPT_HTTPHEADER=>array(
        'Content-Type: application/json',
        'Content-Length: ' . strlen($post)),
    CURLOPT_VERBOSE=>0,
    CURLOPT_RETURNTRANSFER=>true,
    CURLOPT_USERAGENT=>"Mozilla/4.0 (compatible;)",
    CURLOPT_POST=>true,
    CURLOPT_POSTFIELDS=>$post,

);
curl_setopt_array($ch, $options);
$result = curl_exec($ch);
$result=  json_decode($result);
print_r($result);