I cannot retrieve Access Token

API
2

This feels like a convoluted way to do this in PHP.

You most liekly do not need these four lines

			CURLOPT_CAINFO => PATH_TO_CERT, // ssl certificate
			CURLOPT_RETURNTRANSFER => TRUE, // return stuff!
			CURLOPT_SSL_VERIFYPEER => TRUE, // verify peer
			CURLOPT_SSL_VERIFYHOST => 2, // verify host

and here

			// json decode response body
			$apiResponse = json_decode( $apiResponseBody, true );	
		} else { // no headers response is json string
			// json decode response body
			$apiResponse = json_decode( $apiResponse, true );
		}

you assume you get JSON, you might not have done

There is not HTTP Response code checking here either, so you didn’t test if you got a 4xx as apposed to a 2xx code.

This “PHP One Page” example might help you out

Specifically the stuff around line 49 for HTTP Response code checking

		$endpoint = self::TWITCH_ID_DOMAIN . 'oauth2/token';

What is self::TWITCH_ID_DOMAIN set to?

Did you omit a / on the end and it tried to POST to https://id.twitch.tvoauth2/token in error?