警告:session_start(）[function.session start]:无法发送会话cookie-标头已由

我有类似警告的错误：session_start（）[function.session start]：无法发送会话cookie-标头已由（输出在G:''PleskVhosts''globaledudeals.com''translatemasters''list.php:1）在G:'' PleskVhost ''globaledudeals.com'' translatemasters ''list.php的第87行中发送

<?php
ob_start();
session_start(); //**---> This is 87th line**
$message="";
if(count($_POST)>0) {
$conn = mysql_connect("198.72.225.45:3360","test","xxxx");
mysql_select_db("test",$conn);
$result = mysql_query("SELECT * FROM candidate WHERE user_name='" . $_POST["userName"] . "' and pass_word = '". $_POST["password"]."'");
$row  = mysql_fetch_array($result);
if(is_array($row)) {
$_SESSION["user_id"] = $row['id'];
$_SESSION["user_name"] = $row['user_name'];
} else {
$message = "<font color='#FF0000'>Invalid Username or Password!</font>";
}
}
if(isset($_SESSION["user_id"])) {
header("Location:profile.php");
}
ob_flush();
?>