<?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>