通过Curl（Paypal）获取访问令牌

Question

我无法弄清楚遗失的是什么。我想从paypal REST API输出一个示例响应。

Curl example :
curl -v https://api.sandbox.paypal.com/v1/oauth2/token \
-H "Accept: application/json" \
-H "Accept-Language: en_US" \
-u "EOJ2S-Z6OoN_le_KS1d75wsZ6y0SFdVsY9183IvxFyZp:EClusMEUk8e9ihI7ZdVLF5cZ6y0SFdVsY9183IvxFyZp" \
-d "grant_type=client_credentials"

我的代码：

<?php
    define("API_USER","EOJ2S-Z6OoN_le_KS1d75wsZ6y0SFdVsY9183IvxFyZp");
    define("API_PASS","EClusMEUk8e9ihI7ZdVLF5cZ6y0SFdVsY9183IvxFyZp");
    function getAccessToken() {
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
        curl_setopt($ch, CURLOPT_URL, 'https://api.sandbox.paypal.com/v1/oauth2/token');
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_USERPWD, API_USER.':'.API_PASS);
        curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
        curl_setopt($ch, CURLOPT_HTTPHEADER, ("Content-Type: application/json"));
        curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
        curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
        $output = curl_exec($ch);
        echo $output;
    }
?>