:param string $str: the text string to convert
:param bool $protect_all: Whether to protect all content that looks like a potential entity instead of just numbered entities, e.g., &foo;
:returns: XML-converted string
:rtype:    string

Takes a string as input and converts the following reserved XML
characters to entities:

  - Ampersands: &
  - Less than and greater than characters: < >
  - Single and double quotes: ' "
  - Dashes: -

This function ignores ampersands if they are part of existing numbered
character entities, e.g., &#123;. Example:

.. literalinclude:: xml_helper/002.php

outputs:

.. code-block:: html

    &lt;p&gt;Here is a paragraph &amp; an entity (&#123;).&lt;/p&gt;