A simple implementation of a `Maybe` return class in PHP.

Maybe.php

<?php

class Maybe {
	private $value;
	private $error;
	
	private function __construct($value, $error) {
		$this->value = $value;
		$this->error = $error;
	}
	
	public static function fromError($error): Maybe {
		return new Maybe(null, $error);
	}
	
	public static function fromValue($value): Maybe {
		return new Maybe($value, null);
	}
	
	public function isError(): bool {
		return $this->error === null;
	}
	
	public function getError() {
		return $this->error;
	}
	
	public function getValue() {
		return $this->value;
	}
}

Should the default constructor be private?

Yes, it must.