將SimpleXML轉換爲JSON PHP

我正在使用Bigbluebutton的PHP API，並且希望獲得所有會議會話。在調用顯示會議的方法時，我得到以下輸出：將SimpleXML轉換爲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 
      ) 

    ) 

)

如何才能將此simplexml頂級json？

來源

2013-10-04 calamansi_grenade