<?xml version="1.0" encoding="utf-8"?>
<user>
        <person_siteurl>http://www.163.com</person_siteurl>
        <job_keys>PHP</job_keys>
        <job_keys>JAVA</job_keys>
        <job_keys>PYTHON</job_keys>
        <email>jack@sina.com</email>
</user>

$arr = array(
   'person_siteurl'=>'http://www.163.com',
   'job_keys'=>'php,java,python',
    'email'=>'jack@sina.com'
)

$arr=(array)simplexml_load_file('user.xml');
$res=array_map(function($param){return is_array($param)?implode(',',$param):$param;},$arr);
print_r($res);