'Get All Inputs' as raw text

I'm integrating with Segment webhooks. I'd like to implement signature verification according to the Segment docs:

To do this, I'm using 'Get All Inputs' with encoding set to 'none'. I'm expecting to get the raw request body, so that I can apply an hmac_sha1 function against it to check the signature that's provided in the x-signature request header. Unfortunately, it appears that I'm getting an object back, not the raw string, because when I try to apply hmac_sha1, I get this error:

hash_hmac(): Argument #2 ($data) must be of type string, array given

How can I get the entire raw body as an original text string, so that I can use hmac_sha1 with it?

Thanks!