Source

File: wp-includes/rest-api/class-wp-rest-server.php

public function get_headers( $server ) {
		$headers = array();

		// CONTENT_* headers are not prefixed with HTTP_.
		$additional = array(
			'CONTENT_LENGTH' => true,
			'CONTENT_MD5'    => true,
			'CONTENT_TYPE'   => true,
		);

		foreach ( $server as $key => $value ) {
			if ( strpos( $key, 'HTTP_' ) === 0 ) {
				$headers[ substr( $key, 5 ) ] = $value;
			} elseif ( 'REDIRECT_HTTP_AUTHORIZATION' === $key && empty( $server['HTTP_AUTHORIZATION'] ) ) {
				/*
				 * In some server configurations, the authorization header is passed in this alternate location.
				 * Since it would not be passed in in both places we do not check for both headers and resolve.
				 */
				$headers['AUTHORIZATION'] = $value;
			} elseif ( isset( $additional[ $key ] ) ) {
				$headers[ $key ] = $value;
			}
		}

		return $headers;
	}