PHP : cURL is printing the result on it's own without specifying it

Long story short, I'm trying to connect to PayPal sandbox API ,it's getting a response without errors

my issue is it's printing the response without specifying it for some reason, I don't know where that response is stored, it's just inserted automatically in the page

code :

[CONTENT HERE]

$paypal_api_sandbox = 'https://api-m.sandbox.paypal.com/v1/oauth2/token';
$curl = curl_init($paypal_api_sandbox);
curl_setopt($curl, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
curl_setopt($curl,CURLOPT_POSTFIELDS,'grant_type=client_credentials');
curl_setopt($curl , CURLOPT_USERPWD ,'USER:PASS');
$res = curl_exec($curl);

[CONTENT HERE]

fields are :

  USER - My client id 
  PASS - secret key 

   print_r($res) // returns 1

Full output :

[CONTENT]
['content':'PayPal response printed automatically on the page']
[CONTENT]
1

Where's the leak ?