It may be because of extra white spaces after php closing tag (?>) in some file.
Also read this Post:
PHP development: why redirects don't work (headers already sent)
- Any HTML output, including the DOCTYPE declaration or any HTML tag,
including the head of the page
- Extra whitespace before the opening PHP tag of the page, or outside the
PHP tags of an include file
- Using print() or echo before calling header() or session_start()
- Using virtual() to include files
- Using the byte-order mark (BOM) at the beginning of a page
For example:
