$incoming_xml = '
<?xml version="1.0" encoding="UTF-8"?>
<TRANSACTION>
    <FIELDS>
        <FIELD KEY="user">name</FIELD>
        <FIELD KEY="password">pass&word</FIELD>
        <FIELD KEY="operation_type"><do_what></FIELD>
     </FIELDS>
</TRANSACTION>';

##</code><code>##

$converted_xml = '
<?xml version="1.0" encoding="UTF-8"?>
<TRANSACTION>
    <FIELDS>
        <FIELD KEY="user">name</FIELD>
        <FIELD KEY="password">pass!38word</FIELD>
        <FIELD KEY="operation_type">!40do_what!41</FIELD>
     </FIELDS>
</TRANSACTION>';