Curl problem with string and variable？

Guys, I 'v need Curl to post xml to provider's server.
Would you please tell me what is the difference between those codes.

1. Works fine.
   $xml='xml=US';
   $ch = curl_init();
   $header=[ "Content-type: application/x-www-form-urlencoded","charset=utf-8"];
   curl_setopt($ch, CURLOPT_URL, $url);
   curl_setopt($ch, CURLOPT_POST, 1);
   curl_setopt($ch, CURLOPT_HEADER, 0);
   curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
   curl_setopt($ch, CURLOPT_RETURNTRANSFER, 0);
   curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);
   curl_exec($ch);
   
   2.Fail.
   make a txt file contain the characters 'xml=US'.
   
   $xml=file_get_contents("country.txt");
   $ch = curl_init();
   $header=[ "Content-type: application/x-www-form-urlencoded","charset=utf-8"];
   curl_setopt($ch, CURLOPT_URL, $url);
   curl_setopt($ch, CURLOPT_POST, 1);
   curl_setopt($ch, CURLOPT_HEADER, 0);
   curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
   curl_setopt($ch, CURLOPT_RETURNTRANSFER, 0);
   curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);
   curl_exec($ch);
   
   So. what's the difference between $xml one set as charectors and another by file_get_contents?Thank you.