Warning: Cannot modify header information -headers already sent

Problem Description
Whenever you run a php file in the browser it shows the following warning message.
Warning: Cannot modify header information - headers already sent by (output started at E:\xampp\htdocs\mock9\pro01.php:86) in E:\xampp\htdocs\mock9\pro01.php on line 168

A variant of above error is after Warning message a function name is appended and then a error message before displaying "headers already sent".
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at E:\xampp\htdocs\mock9\index.php:4) in E:\xampp\htdocs\mock9\index.php on line 9

Cause and Problem of the Problem
This error "headers already sent" indicates that there are spaces, new lines, other garbage words or in fact any words before an opening tag <?php or after the ending tag ?>.

This same error also occur in fact case 1) if you display any output before sending any header information. Like if you display output like,
echo $query;

before sending header information like before line,
header ("location: pro02.php?pid=$pro_id");
then above error will appear.

It is important to note that with the error message
output started at E:\xampp\htdocs\mock9\pro01.php:86) in E:\xampp\htdocs\mock9\pro01.php on line 168
- It mean that the problem is inside E:\xampp\htdocs\mock9\pro01.php
- The line number that cause the problem is 86 not 168.

Similarly by error message,
output started at E:\xampp\htdocs\mock9\index.php:4) in E:\xampp\htdocs\mock9\index.php on line 9
indicates that,
- Victim file is E:\xampp\htdocs\mock9\index.php
- The problem is in line number 4.

Solution of the Problem

Class concepts and usage syntax in php

Control Structures in php

Track error messages on php

Magic constants in php

Datatypes in Php

Operators in php