PHP Warning: Cannot modify header information - headers already sent

Warning: Cannot modify header information - 
headers already sent by (output started at /php/homepage.php:124)
 in /php/homepage.php on line 145

This is a most common type of error's that once may get in PHP pages with header functions, lets see reasons that could cause this issues and how to fix it,

1. Output Statements:

   You might be having echo, printf output statements before the header that would cause this issue.

2. Unwanted Blank Spaces :

   There should be no blank lines or whitepaces spaces before or after the <php and ?> tags.

3. File Format :

   Make sure that the file format is UTF-8.

4. Include files :

   You might be having a include() to include a php page that may contain header redirects.