You may want to set data in a token header that are not important for your application (e.g. general information). The integrity protection of the data is therefore not needed at all.
The RFC7515 introduces an unprotected header. This header is supported by this framework.
With the example below, we will create a signed token with some unprotected header parameters:
$jws = $jwsBuilder->create()->withPayload('...')->addSignature($jwk, ['alg' => 'HS256'], ['description' => 'Small description here', 'author' => 'John Doe'])->build();
$jws will be a valid JWS object with one signature and both headers.
Note: when an unprotected header is set, the Compact Serialization mode is not available.