在PHP中使用GCM向iOS发送远程推送通知时InternalServerError的结果

我的PHP代码如下：

gcmPush(
    "kU6GAbdQEg0:APA91bGSF42WITzubJ9lwQ2qUqL4ETtZfQEOthazc4tP1CRhxx8tKSRChwgtFK37-kt7RjG2P_Ncj2bDOONpPVsYkjxCNi3NDhMNWSkJ7pnlR83jdbMJfOKlD1CPXCgAvZjbwpzdeiTa","This is only a test from GCM for iOs.","PHP test"
    );

function gcmPush($GCM_regID,$message,$titlE) {

    $GOOGLE_API_KEY = "AIzaSyAd8pw7_J8i-FPswnhUMkFCDHtufSCFFTc";
    $url = 'https://android.googleapis.com/gcm/send';

    $fields = array(
        'to' => $GCM_regID,'notification' => array(
            'sound' => 'default','badge' => '1','body' => $message,'title' => $title
        )
    );

    $headers = array(
        'Authorization: key=' . $GOOGLE_API_KEY,'Content-Type: application/json'
    );

    $ch = curl_init();
    curl_setopt($ch,CURLOPT_URL,$url);
    curl_setopt($ch,CURLOPT_POST,truE);
    curl_setopt($ch,CURLOPT_httpHEADER,$headers);
    curl_setopt($ch,CURLOPT_POSTFIELDS,json_encode($fields));

    $result = curl_exec($ch);
    echo "\r\n";
    if ($result === falSE) {
        die('Failed: ' . curl_error($ch) . "\r\n");
    } else {

        echo "success\r\n";   
    }

    curl_close($ch);
}

请求成功,但Google GCM服务器的结果包含以下内容：

Google Api密钥有什么问题吗？
我尝试使用此脚本将远程通知发送到GcmExample.xcworkspace并且它有效.
我想知道如何解决这个问题.