Question

Why does this give me an error?

$this->_page->page_text = Messages::msg[$explode[1]];

The structure of Messages is

class Messages
{
    public static $msg = array(...);
    public static $ns = array(...);
}

Answer 1

Assuming that $explode is a variable containing some exploded data and you are using the 2nd item in that array as an index for your messsage, you need to use Messages::$msg[$explode[1]];

Variables always start with $