curl模拟POST登陆uc源码

<?php
$a='loginName="$_POST[loginName]"&pwd="$_POST[pwd]"&appId=6&backUrl=http%3A%2F%2Fuser.9game.cn%2FLoginRedirect.aspx%3FrtnUrl%3Dhttp%253a%252f%252fwap.9game.cn%252fpage%252fsymbian%252fpage%252f3years.html%253ftext%253dsymbian2_android_client%2526positionid%253d39%2526adsid%253d803%2526tm%253d2%2526SessionId%253d%2526Clear%253d1&display=&scope=9game_scope&actType=&imageField_x=10&imageField_y=12';
$ch=curl_init();
$url="http://login.uc.cn/login/commit?appId=6";
curl_setopt($ch,CURLOPT_HEADER,1);
curl_setopt($ch,CURLOPT_URL,$url);
curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
  curl_setopt($ch,CURLOPT_POST,1);
  curl_setopt($ch,CURLOPT_POSTFIELDS,$a);
   $data=curl_exec($ch);
 curl_close($ch);
 echo var_dump($data);
echo '<div style="display:none">';
 ?>这个源码应用后，返回的页面会提示账户密码都为空，不管我把POST数据做成数组还是http_build_query()后，这个怎么回事？@老虎会游泳@爱特 求教，谢了