how can i use fopen() with CodeIgniter?

[eluser]chengfeng[/eluser]
Hi, i am trying to open text file via URL, but the error occurred,it said:
Message: fopen(http://www.example.com/text.txt) [function.fopen]: failed to open stream: HTTP request failed! HTTP/1.1 500 Internal Server Error
What should i do? Please help!
Note that if i use fopen() without CI , it's worked, no error occurred. Just simple code.
The code is:
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$filename="http://www.example.com/text.txt";
$file=fopen($filename,"r");

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Thank you indeed!