PHP - Cannot Send Session Cookie - Headers Already Sent

I am confused why I am getting the following error and was wondering if I could get a 2nd pair of eyes..
error:
[29-Jul-2010 20:58:01] PHP Warning:  session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cookie - headers already sent in /home/curren11/public_html/mysite***.com/file_monitor.php on line 2
[29-Jul-2010 20:58:01] PHP Warning:  session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cache limiter - headers already sent (output started at /home/curren11/public_html/mysite***.com/file_monitor.php:2) in /home/curren11/public_html/mysite***.com/file_monitor.php on line 2

Here is the code:
<?php
session_start();

$root_path = realpath(getcwd());
$error_monitor_file = $root_path.'/file_modify.txt';
$hour = date("H");
$today = date("m-d-Y");



     $myFile2 = "file_monitor.php.log";
     $fh3 = fopen($myFile2, 'a');
     $log_msg = "Last ran on: ".$today." at: ".$hour." \n";
     fwrite($fh3, $log_msg);
    

function scan_dir($path)
{
 $out = array();
 $files = opendir($path);

while ($f = readdir($files))
{
  $extension = substr($f,-3);
     
   if ($f=='.' or $f=='..' or $extension=='jpg' or $extension=='JPG' or $extension=='png' or $extension=='gif' or $f=='file_monitor.php' or $f=='file_modify.txt' or $f=='file_monitor.php.log' or $f =='error_log'  ) 
     {
   continue;
 }
 
  if (is_dir($path.'/'.$f))
    {
  $out = array_merge($out, scan_dir($path.'/'.$f));
} 
  //if  (((!is_dir($path.'/'.$f)) && ($f!='.' or $f!='..' or $extension!='jpg' or $extension!='JPG' or $extension!='png' or $extension!='gif'))
  else
  //  {
    $out[] = $path. '/'. $f;
//}
}

  $_SESSION['all_files'] = $out;
  
 return $out;
}

$a = scan_dir(getcwd());

$count_array = count($_SESSION['all_files']);
$loop_count=0;
while($loop_count < $count_array)
{
  // file in directory when was it last changed ?
  $filechange1 = date("m-d-Y-H", filemtime($_SESSION['all_files'][$loop_count]));
 
 // was it chnaged today and have I sent out an alert yet?
 if($filechange1 == $today."-".$hour) 
  {
$fh2 = fopen($error_monitor_file, 'a');
fwrite($fh2, "\n alert, file: ".$_SESSION['all_files'][$loop_count]." was changed today within the last hour ! \n");
    fclose($fh2);
  }
 $loop_count ++;
 // log file? was it changed this past hour?
 if (file_exists($error_monitor_file))
 {
   $filechange_log = date("m-d-Y-H", filemtime($error_monitor_file));
 
if ( ($loop_count == $count_array ) && ($filechange_log == $today."-".$hour ) )
{
        email();
     }
}
}

// function for email...
function email()
{
require_once ('class.phpmailer.php');

$mail = new PHPMailer();

$mail->IsSMTP();                                   // send via SMTP
$mail->Host     = "localhost"; // SMTP servers
$mail->SMTPAuth = true;     // turn on SMTP authentication
$mail->Username = "**c66@***ar.com";  // SMTP username
$mail->Password = "*****"; // SMTP password
$mail->From     = "***@***.com";
$mail->FromName = "Colonial Cottage File Changed";
$mail->AddAddress("email@*****.com"); 

$mail->WordWrap = 50;                              // set word wrap
//$mail->AddAttachment("/var/tmp/file.tar.gz");      // attachment
//$mail->AddAttachment("/tmp/image.jpg", "new.jpg"); 
$mail->IsHTML(true);                               // send as HTML

$mail->Subject  ="Unauthorized file changed today!";
$mail->Body     =  "" ;

$mail->AltBody  =  "This is the text-only body";

if(!$mail->Send())
   {
     exit();
   }
}

 if ((file_exists($error_monitor_file)) && $hour == "00")
 {
   unlink($error_monitor_file);
 }
 fclose($fh3);
 exit();
?>