You use the header() function. You can not send any output to the browser before using this function. And your <body> is an output.
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.
