@adilalima, como vai?
Na SDK o parâmetro 'x-skip-mtls-checking' deve ser passado no array 'headers'. Assim:
php
$webhook = EfiPay::getInstance([
'client_id' => CONF_EFI_SANDBOX ? CONF_EFI_DEV_CLIENT_ID : CONF_EFI_CLIENT_ID,
'client_secret' => CONF_EFI_SANDBOX ? CONF_EFI_DEV_CLIENT_SECRET : CONF_EFI_CLIENT_SECRET,
'certificate' => CONF_EFI_SANDBOX ? CONF_EFI_DEV_PIX_CERTIFICATE : CONF_EFI_PIX_CERTIFICATE,
'headers' => ['x-skip-mtls-checking' => "true"],
]);