PHP printed this

Warning: Cannot modify header information - headers already sent by

What it meansYour script tried to send a header or cookie after output had already started, and headers must go out before any body content.

The first thing to checkThe warning names the file and line where output began; look for whitespace before <?php, a BOM, an echo, or a closing ?> at the end of a file.

The walrus saysA single stray space before the opening tag. Always.