php gets xml data submitted via http protocol post and parses xml

How does php get the requested xml data, xml data submitted by the other party via http protocol post, and php get the data?

 
<?php 
$xml_data ='<AATAvailReq1>'. 
'<Agency>'. 
'<Iata>1234567890</Iata>'. 
'<Agent>lgsoftwares</Agent>'. 
'<Password>mypassword</Password>'. 
'<Brand>phpmind.com</Brand>'. 
'</Agency>'. 
'<Passengers>'. 
'<Adult AGE="" ID="1"></Adult>'. 
'<Adult AGE="" ID="2"></Adult>'. 
'</Passengers>'. 
'<HotelAvailReq1>'. 
'<DestCode>JHM</DestCode>'. 
'<HotelCode>OGGSHE</HotelCode>'. 
'<CheckInDate>101009</CheckInDate>'. 
'<CheckOutDate>101509</CheckOutDate>'. 
'<UseField>1</UseField>'. 
'</HotelAvailReq1>'. 
'</AATAvailReq1>'; 
$URL = "https://www.yourwebserver.com/path/"; 
$ch = curl_init($URL); 
curl_setopt($ch, CURLOPT_MUTE, 1); 
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0); 
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0); 
curl_setopt($ch, CURLOPT_POST, 1); 
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: text/xml')); 
curl_setopt($ch, CURLOPT_POSTFIELDS, "$xml_data"); 
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1); 
$output = curl_exec($ch); 
curl_close($ch); 
?>