gistfile1.php

<?php

class error {
    public $title;
    public $code;
    public $contains = array();
    
    public function __construct($title, $code = 0, Array $contains = array()) {
        $this->title = $title;
        $this->code = $code;
        $this->contains = $contains;
    }
}

class object {
    public $foo;
    public $baz;

    public function validate(Error &$error = null) {
        $errors = array();

        if (!$this->foo) {
            $errors[] = new error('Foo not set');
        }
        if (!$this->baz) {
            $errors[] = new error('blah');
        }
        
        if (empty($errors)) {
            return true;
        }

        $error = count($errors) == 1 ? $errors[0] : new error('multiple errors', 0, $errors);
        return false;
    }
    
    public function loadModule($module) {
        throw new exception('module not found: ' . $module);
    }
}

$object = new object;
$error = null;
var_dump($object->validate($error));
var_dump($error);

$object->foo = 'baz';
$object->baz = 'bam';
$error = null;
var_dump($object->validate($error));
var_dump($error);