WP_HTTP_Response::header( string $key, string $value, bool $replace = true )

Source

File: wp-includes/class-wp-http-response.php

public function header( $key, $value, $replace = true ) {
		if ( $replace || ! isset( $this->headers[ $key ] ) ) {
			$this->headers[ $key ] = $value;
		} else {
			$this->headers[ $key ] .= ', ' . $value;
		}
	}