Função start com erro

[Lúcio Marques]

Bom gente, boa noite.

Tenho uma função criada pelo dreamweaver que faz a função de start - restrinção de usuario. No servidor wamp5 funciona em meu pc, mas quando jogo para o servidor da web fica mostrando o seguinte erro:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/procarve/public_html/admin/usuariopainel.php:1) in /home/procarve/public_html/admin/usuariopainel.php on line 4

Warning: Cannot modify header information - headers already sent by (output started at /home/procarve/public_html/admin/usuariopainel.php:1) in /home/procarve/public_html/admin/usuariopainel.php on line 43

Gostaria de saber o que pode ser, já coloque outro site e funcionou perfeitamente mas com o atual não está dando certo. O código de restrinção que usso é esse:

<?php
if (!isset($_SESSION)) {
  session_start();
}
$MM_authorizedUsers = "";
$MM_donotCheckaccess = "false";

// *** Restrict Access To Page: Grant or deny access to this page
function isAuthorized($strUsers, $strGroups, $UserName, $UserGroup) { 
  // For security, start by assuming the visitor is NOT authorized. 
  $isValid = False; 

  // When a visitor has logged into this site, the Session variable MM_Username set equal to their username. 
  // Therefore, we know that a user is NOT logged in if that Session variable is blank. 
  if (!empty($UserName)) { 
    // Besides being logged in, you may restrict access to only certain users based on an ID established when they login. 
    // Parse the strings into arrays. 
    $arrUsers = Explode(",", $strUsers); 
    $arrGroups = Explode(",", $strGroups); 
    if (in_array($UserName, $arrUsers)) { 
      $isValid = true; 
    } 
    // Or, you may restrict access to only certain users based on their username. 
    if (in_array($UserGroup, $arrGroups)) { 
      $isValid = true; 
    } 
    if (($strUsers == "") && false) { 
      $isValid = true; 
    } 
  } 
  return $isValid; 
}

$MM_restrictGoTo = "login.php";
if (!((isset($_SESSION['MM_Username'])) && (isAuthorized("",$MM_authorizedUsers, $_SESSION['MM_Username'], $_SESSION['MM_UserGroup'])))) {   
  $MM_qsChar = "?";
  $MM_referrer = $_SERVER['PHP_SELF'];
  if (strpos($MM_restrictGoTo, "?")) $MM_qsChar = "&";
  if (isset($QUERY_STRING) && strlen($QUERY_STRING) > 0) 
  $MM_referrer .= "?" . $QUERY_STRING;
  $MM_restrictGoTo = $MM_restrictGoTo. $MM_qsChar . "accesscheck=" . urlencode($MM_referrer);
  header("Location: ". $MM_restrictGoTo); 
  exit;
}
?>

Editado Novembro 24, 2010 por Lúcio Marques

[svjets]

Tente colocar "ob_start();", sem as aspas como primeiro comando do script.