The keys represent the header name as it will be sent over the wire, and
each value is an array of strings associated with the header.
Represent the headers as a string
foreach ($message->getHeaders() as $name => $values) {
echo $name . ": " . implode(", ", $values);
}
Emit headers iteratively:
foreach ($message->getHeaders() as $name => $values) {
foreach ($values as $value) {
header(sprintf('%s: %s', $name, $value), false);
}
}