Why do i get the error "Cannot add header information" ?

Remember that header() must be called before any actual output is
sent, either by normal HTML tags blank lines in a file, or from
PHP.

It is a very common error to read code with include(), or
require(), functions, or another file access function, and have spaces
or empty lines that are output before header() is called. The same
problem exists when using a single PHP/HTML file.