“警告:session_start()[function.session-start]:无法发送会话cookie-标头已发送"在文件中首先调用session_start()时

<?php
   session_start();
   $_SESSION['start']=1;

   echo $_SESSION['start'];
?>

以FF输出:

警告:session_start() [function.session-start]:无法发送 会话Cookie-标头已发送 通过(输出从.../test.php:1开始) 在第10行的.../test.php中

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at .../test.php:1) in.../test.php on line 10

警告:session_start() [function.session-start]:无法发送 会话缓存限制器-标头 已发送者(输出始于 .../test.php:1)在.../test.php中 10

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent by (output started at .../test.php:1) in.../test.php on line 10