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

Question

This question already has answers here:
How to fix "Headers already sent" error in PHP
(11 answers)
Reference - What does this error mean in PHP?
(38 answers)
Closed 9 years ago.
<?php session_start();
include_once("include/Connection.php");
$userid=$_POST['userid'];
$password=$_POST['password'];
$loginSql = "SELECT * FROM admin2 WHERE admin_name='$userid' and admin_psw='$password'";
$loginQuery=mysql_query($loginSql);
$num_row = mysql_num_rows($loginQuery);
$login = mysql_fetch_array($loginQuery);
$user = $login['admin_name'];
$password = $login['admin_psw'];
echo $user;
echo $password;
if($num_row >0)
{
if($user == $userid)
{
$_SESSION['id_gosi'] = $login['id'];
$_SESSION['userid_gosi'] = $login['admin_name'];
header("Location: CustomerMaster.php");
}
else{
header("Location: index.php?msg=2");
exit();}}
else{
header("Location: index.php?msg=1");
}?>
Warning: Cannot modify header information - headers already sent by (output started at C:\wamp\www\php_mahavirbhadra\Admin\validate_login.php:17) in C:\wamp\www\php_mahavirbhadra\Admin\validate_login.php on line 26
Warning: Unknown: 1 result set(s) not freed. Use mysql_free_result to free result sets which were requested using mysql_query() in Unknown on line 0
when i write above code for admin login in php than connection successfully done but give me above error what can i do for this error ...Please give me solution....

Answer 1

print/echo before HEADER is WRONG !!!
/* echo $user;
echo $password;*/
if($num_row >0)
{
if($user == $userid)
{
....
header("Location: CustomerMaster.php");
}
else
{
header("Location: index.php?msg=2");
exit();
}
}
else
header("Location: index.php?msg=1");
?>
never echo ANYTHING before HEADER, so remove those ECHO $USER, $PASSWORD
Tip 1-> Use mysqli instead of mysql
Tip 2-> Use prepared statement
Tip 3-> Use mysql_real_escape_string()