The following code will add product to a cart/quote for a logged-in customer.
<?php
$magentoURL = "http://127.0.0.1/mage2/";
$apiURL = $magentoURL."index.php/rest/V1/integration/customer/token";
$data = array("username" => "Test.User1521623008@phpcodez.com", "password" => "phpcodezpwd");
$data_string = json_encode($data);
$ch = curl_init($apiURL);
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);
$quoteUrl =$magentoURL.'index.php/rest/V1/carts/mine';
$chQuote = curl_init($quoteUrl);
curl_setopt($chQuote, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Authorization: Bearer " . json_decode($token)));
curl_setopt($chQuote, CURLOPT_RETURNTRANSFER, true);
$quote = json_decode(curl_exec($chQuote));
$cartData = [
'cartItem' => [
"quote_id" => $quote->id,
"sku" => "152110360696",
"qty" => 2
]
];
$requestUrl =$magentoURL.'index.php/rest/V1/carts/mine/items';
$ch = curl_init($requestUrl);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($cartData));
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Authorization: Bearer " . json_decode($token)));
$result = json_decode(curl_exec($ch));
echo "<pre>";print_r($result);
?>