Turn API Return to Object PHP

My API returns a JSON that I would like to transform into a PHP object so that I can find a specific field in return. how can I do this?

My get

public function teste()
{
    $requestUrl='http://XXXXXX:XXX/api/login';
    $users = array("wts-authorization: XXX/XXx","content-type: application/json","accept: application/json");
    $ch = curl_init($requestUrl);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $users);
    $obj = curl_exec($ch);

}

My return

{"page":2,"per_page":6,"total":12,"total_pages":2,"data":[{"id":7,"email":"michael.lawson@reqres.in","first_name":"Michael","last_name":"Lawson","avatar":"https://s3.amazonaws.com/uifaces/faces/twitter/follettkyle/128.jpg"},{"id":8,"email":"lindsay.ferguson@reqres.in","first_name":"Lindsay","last_name":"Ferguson","avatar":"https://s3.amazonaws.com/uifaces/faces/twitter/araa3185/128.jpg"},{"id":9,"email":"tobias.funke@reqres.in","first_name":"Tobias","last_name":"Funke","avatar":"https://s3.amazonaws.com/uifaces/faces/twitter/vivekprvr/128.jpg"},{"id":10,"email":"byron.fields@reqres.in","first_name":"Byron","last_name":"Fields","avatar":"https://s3.amazonaws.com/uifaces/faces/twitter/russoedu/128.jpg"},{"id":11,"email":"george.edwards@reqres.in","first_name":"George","last_name":"Edwards","avatar":"https://s3.amazonaws.com/uifaces/faces/twitter/mrmoiree/128.jpg"},{"id":12,"email":"rachel.howell@reqres.in","first_name":"Rachel","last_name":"Howell","avatar":"https://s3.amazonaws.com/uifaces/faces/twitter/hebertialmeida/128.jpg"}]}

I would like to treat as an object and use as follows

obj->first_name == “Michael”

Magento 2.3.3
Win10 Xampp