"Warning: Cannot modify header information - headers already sent by" error [duplicate]

I keep receiving this error each time I try to submit a deletion form.

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\speedycms\deleteclient.php:47) in C:\xampp\htdocs\speedycms\deleteclient.php on line 106

Is there something wrong with my code? What do I need to change to make it work?

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

// *** 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 == "") && true) { 
      $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;
}
?>

<?php
    require_once('Connections/speedycms.php');
    
    $client_id = mysql_real_escape_string($_GET['id']); 
            
    $con = mysql_connect($hostname_speedycms, $username_speedycms, $password_speedycms);

if (!$con)
  {
  die('Could not connect: ' . mysql_error());
  }

mysql_select_db("speedycms") or die(mysql_error());
?>

<?php
if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "") 
{
  if (PHP_VERSION < 6) {
    $theValue = get_magic_quotes_gpc() ? stripslashes($theValue) : $theValue;
  }

  $theValue = function_exists("mysql_real_escape_string") ? mysql_real_escape_string($theValue) : mysql_escape_string($theValue);

  switch ($theType) {
    case "text":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;    
    case "long":
    case "int":
      $theValue = ($theValue != "") ? intval($theValue) : "NULL";
      break;
    case "double":
      $theValue = ($theValue != "") ? doubleval($theValue) : "NULL";
      break;
    case "date":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;
    case "defined":
      $theValue = ($theValue != "") ? $theDefinedValue : $theNotDefinedValue;
      break;
  }
  return $theValue;
}
}

if ((isset($_GET['id'])) && ($_GET['id'] != "") && (isset($_POST['deleteForm']))) {
  $deleteSQL = sprintf("DELETE FROM tbl_accident WHERE id=%s",
                       GetSQLValueString($_GET['id'], "int"));

  mysql_select_db($database_speedycms, $speedycms);
  $Result1 = mysql_query($deleteSQL, $speedycms) or die(mysql_error());

  $deleteGoTo = "progress.php";
  if (isset($_SERVER['QUERY_STRING'])) {
    $deleteGoTo .= (strpos($deleteGoTo, '?')) ? "&" : "?";
    $deleteGoTo .= $_SERVER['QUERY_STRING'];
  }
  header(sprintf("Location: %s", $deleteGoTo));
}

mysql_select_db($database_speedycms, $speedycms);
$query_delete = "SELECT * FROM tbl_accident  WHERE id=$client_id";
$delete = mysql_query($query_delete, $speedycms) or die(mysql_error());
$row_delete = mysql_fetch_assoc($delete);
$totalRows_delete = mysql_num_rows($delete);
?>

<p class="form2">Are you sure you wish to <b>delete</b> the record for <?php echo $row_delete['clientName']; ?>?</p>
                <form name="form" method="POST" action="<?php echo $deleteAction; ?>">
            <p class="form2"><input type="submit" value="Yes" />
              <input name="no" type="button" id="no" value="No" />
            </p>
                              <input type="hidden" name="deleteForm" value="form" />
        </form>     

This typically occurs when there is unintended output from the script before you start the session. With your current code, you could try to use output buffering to solve it.

try adding a call to the ob_start(); function at the very top of your script and ob_end_flush(); at the very end of the document.

Lines 45-47:

?>

<?php

That's sending a couple of newlines as output, so the headers are already dispatched. Just remove those 3 lines (it's all one big PHP block after all, no need to end PHP parsing and then start it again), as well as the similar block on lines 60-62, and it'll work.

Notice that the error message you got actually gives you a lot of information to help you find this yourself:

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\speedycms\deleteclient.php:47) in C:\xampp\htdocs\speedycms\deleteclient.php on line 106

The two bolded sections tell you where the item is that sent output before the headers (line 47) and where the item is that was trying to send a header after output (line 106).