class spintxt { 
    public function process($text) { 
        return preg_replace_callback(
            '/\{(((?>[^\{\}]+)|(?R))*)\}/x',
            array($this, 'replace'),
            $text
        );
    }
    public function replace($text) { 
        $text = $this->process($text[1]);
        $parts = explode('|', $text);
        return $parts[array_rand($parts)];
    }
}

$spintxt = new spintxt();
$string  = '{Hello|Hi}, my name is {Jack|Harry|Lucy|Alex|Mary}';
echo $spintxt->process($string);