PHP Header
Header() must be called before any output is sent. This includes with the standard HTML tags, empty lines, or output from PHP.
It is common error to read code with require(), or include(), functions, which results in output before header() is called. Likewise, the problem exists when a single PHP/HTML file is used.
It is common error to read code with require(), or include(), functions, which results in output before header() is called. Likewise, the problem exists when a single PHP/HTML file is used.