Translate command-line cURL to PHP

I have a working cURL request I want to translate to PHP code:

curl \
    -X POST \
    -u live_abcdefg: \
    -d '{
        "recipient": {
            "address": "[email protected]"
        }
    }' \
    https://api.sendwithus.com/api/v1/drip_campaigns/abcdefgh/activate
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Here is the PHP code I'm trying:

<?php

$theurl = "https://api.sendwithus.com/api/v1/drip_campaigns/abcdefgh/activate";

$ch = curl_init($theurl);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'POST'); // -X
curl_setopt($ch, CURLOPT_POSTFIELDS, '{ "recipient": { "address": "[email protected]" } }'); // -d
curl_setopt($ch, CURLOPT_PROXYUSERPWD, 'live_abcdefg:'); // -u
$response = curl_exec($ch);

var_dump($response); // prints bool(true)

?>
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EDIT: Sendwithus (the app that I'm sending requests to) shows that the second request didn't come through (second recipient [email protected] wasn't saved).
What am I doing wrong?
What can I change in the PHP code to start debugging this issue?