--- /dev/null
+# http\Message http\Message::setHeader(string $header[, mixed $value = NULL])
+
+Set a single header.
+See http\Message::getHeader() and http\Message::addHeader().
+
+## Params:
+
+* string $header
+ The header's name.
+* Optional mixed $value = NULL
+ The header's value. Removes the header if NULL.
+
+## Returns:
+
+* http\Message, self.