$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $api_root);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, "xml"); // tried http_build_query also
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); // Added this, still no good
return curl_exec($ch);  // returns false

$curl = "curl -X POST -d 'xml' {$api_root}";
return `$curl`;  // returns expected xml from server

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

$curl = "curl -X POST -d 'xml' {$api_root}";
return `$curl`;  // returns expected xml from server

// Thanks to all the answers
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

// This appears to default false on CLI, true in libcurl
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);