?php
$doc = new DOMDocument('1.0','utf-8');
$doc->formatOutput = true;
//创建标签
$mysql = $doc->createElement("mysql");
$host = $doc->createElement("host");
$username = $doc->createElement("username");
$password = $doc->createElement("password");
$database = $doc->createElement("database");
//创建标签内容
$hostval = $doc->createTextNode("127.0.0.1");
$usernameval = $doc->createTextNode("root");
$passwordval = $doc->createTextNode("1234");
$databaseval = $doc->createTextNode("test");
//绑定标签和内容
$host->appendChild($hostval);
$username->appendChild($usernameval);
$password->appendChild($passwordval);
$database->appendChild($databaseval);
//关联标签之间的关系
$doc->appendChild($mysql);
$mysql->appendChild($host);
$mysql->appendChild($username);
$mysql->appendChild($password);
$mysql->appendChild($database);
$doc->save("config.xml");

?xml version="1.0" encoding="utf-8"?>
mysql>
 host>127.0.0.1/host>
 username>root/username>
 password>1234/password>
 database>test/database>
/mysql>

?php
$mysql = new SimpleXMLElement('?xml version="1.0" encoding="utf-8"?>mysql>/mysql>');
$host = $mysql->addchild("host","127.0.0.1");
$host->addAttribute("note","localhost");
$mysql->addchild("username","root");
$mysql->addchild("password","1234");
$mysql->addchild("database","test");
header("Content-type:text/xml;charset=utf-8");
echo $mysql->asXml();
$mysql->asXml("config.xml");

mysql>
host note="localhost">127.0.0.1/host>
username>root/username>
password>1234/password>
database>test/database>
/mysql>