class Demo
{
    protected $example = 'original value';

    function & example()
    {
        return $this->example;
    }
}

$demo = new Demo;

#1 - pass by value

$var1 = $demo->example();
$var1 = 'var1 value';

echo '$demo->example() => '.$demo->example().PHP_EOL;
echo PHP_EOL;

#2 - pass by reference

$var2 =& $demo->example();
$var2 = 'var2 value';

echo '$demo->example() => '.$demo->example().PHP_EOL;
echo '$var1 => '.$var1.PHP_EOL;
echo '$var2 => '.$var2.PHP_EOL;
echo PHP_EOL;

#3 - altering other references

$var3 =& $demo->example();
$var3 = 'var3 value';

echo '$demo->example() => '.$demo->example().PHP_EOL;
echo '$var1 => '.$var1.PHP_EOL;
echo '$var2 => '.$var2.PHP_EOL;
echo '$var3 => '.$var3.PHP_EOL;
echo PHP_EOL;