php – 提供的密钥参数不能通过Google API强制转换为私钥

表达的所有常量都有正确的值,路径正确且没有空内容.

我得到的错误：

openssl_sign(): supplied key param cannot be coerced into a private key

我实现的功能是：

public static function storageURL( $id,$method = 'GET',$duration = 10 ) {
    $key = file_get_contents(self::KEY_FILE);
    $pkey = openssl_get_privatekey($key,'notasecret');
    $expires = time( ) + $duration;
    $content_type = ($method == 'PUT') ? 'application/x-www-form-urlencoded' : ''; 
    $to_sign = ($method . "n" . 
    /* Content-MD5 */ "n" . 
    $content_type . "n" . 
    $expires . "n" . 
    '/'.self::BUCKET_NAME.'/' . $id); 
    $signature = '*Signature will go here*';
    if (!openssl_sign( $to_sign,$signature,$pkey,'sha256' ))
    { 
        error_log( 'openssl_sign failed!' );
        $signature = '<failed>'; 
    } else { 
        $signature = urlencode( base64_encode( $signature ) ); 
    } 
    return ('https://'.self::BUCKET_NAME.'.commondatastorage.googleapis.com/' .
        $id .
        '?GoogleAccessId=' . self::SERVICE_ACCOUNT_NAME .
        '&Expires=' . $expires . '&Signature=' . $signature);
    }