无法将生日转换为字符串 facebook php sdk

我正在测试facebook php sdk。我拥有"user_birthday，电子邮件"和默认用户配置文件的权限。这是代码。

<?php
if(isset($session)) {
  try {
$user_profile = (new FacebookRequest(
  $session, 'GET', '/me/'
))->execute()->getGraphObject(GraphUser::className());
   var_dump ($user_profile);
   echo $user_profile->getEmail();
   $_SESSION['user']=$user_profile->getName();
   $_SESSION['user_id']=$user_profile->getId();
    $_SESSION['profile_pic']="http://graph.facebook.com/".$_SESSION['user_id']."/picture";
   $_SESSION['gender']=$user_profile->getGender();
 $vars = get_object_vars ( $user_profile->getBirthday() );

 print_r ( $vars );

 } catch(FacebookRequestException $e) {
echo "Exception occured, code: " . $e->getCode();
echo " with message: " . $e->getMessage();
  }   
}        
?>

输出是

    object(Facebook'GraphUser)[6]
    protected 'backingData' => 
    array (size=12)
    'id' => string 'xxxxxxxxxxx' (length=15)
    'birthday' => string '01/12/1990' (length=10)
    'email' => string 'xxxxx@gmail.com' (length=19)
    'first_name' => string 'xxxx' (length=10)
    'gender' => string 'male' (length=4)
    'last_name' => string 'xxxx' (length=4)
    'link' => string 'https://www.facebook.com/app_scoped_user_id/xxxxxx/' (length=60)
    'locale' => string 'en_US' (length=5)
    'name' => string 'xxxxx xxxxx' (length=15)
    'timezone' => int -8
    'updated_time' => string '2015-02-22T07:48:23+0000' (length=24)
    'verified' => boolean true
     xxxxx@gmail.comArray ( [date] => 1993-04-22 00:00:00 [timezone_type] => 3 [timezone] => UTC ) 

问题是，我无法使用生日对象。当我用回声检查生日时，

    echo $user_profile->getBirthday();

我收到此错误。

     Catchable fatal error: Object of class DateTime could not be converted to string

当我转储$user_profile->getBirthday（）;输出是。

      object(DateTime)[9]
      public 'date' => string '1993-04-22 00:00:00' (length=19)
      public 'timezone_type' => int 3
      public 'timezone' => string 'UTC' (length=3)

如何获得字符串形式的生日？ 我是 php 的新手。 任何帮助将不胜感激。