将SimpleXML转换为JSON PHP

我正在使用Bigbluebutton的PHP API，并且希望获得所有会议会话。在调用显示会议的方法时，我得到以下输出：将SimpleXML转换为JSON PHP

Array (

[returncode] => SimpleXMLElement Object 
    (
     [0] => SUCCESS 
    ) 

[messageKey] => SimpleXMLElement Object 
    (
    ) 

[message] => SimpleXMLElement Object 
    (
    ) 

[0] => Array 
    (
     [meetingId] => SimpleXMLElement Object 
      (
       [0] => as's meeting 
      ) 

     [meetingName] => SimpleXMLElement Object 
      (
       [0] => as's meeting 
      ) 

     [createTime] => SimpleXMLElement Object 
      (
       [0] => 1380878550574 
      ) 

     [attendeePw] => SimpleXMLElement Object 
      (
       [0] => ap 
      ) 

     [moderatorPw] => SimpleXMLElement Object 
      (
       [0] => mp 
      ) 

     [hasBeenForciblyEnded] => SimpleXMLElement Object 
      (
       [0] => false 
      ) 

     [running] => SimpleXMLElement Object 
      (
       [0] => false 
      ) 

    ) 

[1] => Array 
    (
     [meetingId] => SimpleXMLElement Object 
      (
       [0] => XYZ's meeting 
      ) 

     [meetingName] => SimpleXMLElement Object 
      (
       [0] => XYZ's meeting 
      ) 

     [createTime] => SimpleXMLElement Object 
      (
       [0] => 1380879253000 
      ) 

     [attendeePw] => SimpleXMLElement Object 
      (
       [0] => ap 
      ) 

     [moderatorPw] => SimpleXMLElement Object 
      (
       [0] => mp 
      ) 

     [hasBeenForciblyEnded] => SimpleXMLElement Object 
      (
       [0] => false 
      ) 

     [running] => SimpleXMLElement Object 
      (
       [0] => true 
      ) 

    ) 

)

如何才能将此simplexml顶级json？

来源

2013-10-04 calamansi_grenade