php对xml的操作十分简单，

$url = 'http://api.map.baidu.com/telematics/v3/weather?location=北京&output=xml&ak=gfPnxaaiPyNIhM7c3eZ1kmEL';
$xml = simplexml_load_file($url);

echo $city = $xml->xpath('//currentCity')[0];
foreach($xml->xpath('//weather_data') as $r) {
  $date = (array)$r->date;
  $dayPictureUrl = (array)$r->dayPictureUrl;
}
var_dump($city, $date, $dayPictureUrl);

北京object(SimpleXMLElement)#2 (0) {
}
array(4) {
  [0]=>
  string(30) "周五(今天, 实时：24℃)"
  [1]=>
  string(6) "周六"
  [2]=>
  string(6) "周日"
  [3]=>
  string(6) "周一"
}
array(4) {
  [0]=>
  string(57) "http://api.map.baidu.com/images/weather/day/leizhenyu.png"
  [1]=>
  string(51) "http://api.map.baidu.com/images/weather/day/yin.png"
  [2]=>
  string(52) "http://api.map.baidu.com/images/weather/day/qing.png"
  [3]=>
  string(52) "http://api.map.baidu.com/images/weather/day/qing.png"
}