Web Programming Step by Step, 2nd Edition

Lecture 24: Web Services

Reading: none

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

What is "Web 2.0"?

• Web 2.0: A set of ideas and technologies for creating modern, interactive web applications
  • Ajax, multimedia, streaming, stateful pages, cookies, user-generated content, web services, ...

What is a web service?

web service: software functionality that can be invoked through the internet using common protocols

• like a remote function(s) you can call by contacting a program on a web server
• many web services accept parameters and produce results
• can be written in PHP and contacted by the browser in HTML and/or Ajax code
• service's output might be HTML but could be text, XML, JSON or other content

Setting content type with header

header("Content-type: type/subtype");

header("Content-type: text/plain");
print "This output will appear as plain text now!\n";

• by default, a PHP file's output is assumed to be HTML (text/html)
• use the header function to specify non-HTML output
  • must appear before any other output generated by the script

Recall: Content ("MIME") types

• Lists of MIME types: by type, by extension

Example: Exponent web service

• Write a web service that accepts a base and exponent and outputs base raised to the exponent power. For example, the following query should output 81 :

  http://example.com/exponent.php?base=3&exponent=4
  

• solution:

  <?php
  header("Content-type: text/plain");
  $base = (int) $_GET["base"];
  $exp  = (int) $_GET["exponent"];
  $result = pow($base, $exp);
  print $result;
  ?>
  

Using header to redirect between pages

header("Location: url");

if (!user_is_logged_in()) {
	header("Location: user-login.php");
}

• you can use header to tell the browser to redirect itself to another page
  • must appear before any other output generated by the script

Exercise: Baby name web service

• Write a web service that accepts a name and gender and finds and outputs the line from text file rank.txt with information about that name:

  Aaron m 147 193 187 199 250 237 230 178 52 34 34 41 55
  Lisa f 0 0 0 0 0 733 220 6 2 16 64 295 720
  ...
  

• For the following call:

  http://example.com/babynames.php?name=Lisa&gender=f
  

• The service should output the following line:

  Lisa f 0 0 0 0 0 733 220 6 2 16 64 295 720
  

What about errors?

• What if the user doesn't pass an important parameter?

  http://example.com/babynames.php?gender=f               (no name passed!)
  

• What if the user passes a name that is not found in the file?

  http://example.com/babynames.php?name=Borat&gender=m    (not found in file)
  

• What is the appropriate behavior for the web service?

Reporting errors

• web service should return an HTTP "error code" to the browser, possibly followed by output
  • error messages (print) are not ideal, because they could be confused for normal output
  • these are the codes you see in Firebug's console and in your Ajax request's status property

  HTTP code	Meaning
  200	OK
  301-303	page has moved (permanently or temporarily)
  400	illegal request
  401	authentication required
  403	you are forbidden to access this page
  404	page not found
  410	gone; missing data or resource
  500	internal server error
  complete list

Using headers for HTTP error codes

header("HTTP/1.1  code  description");

if ($_GET["foo"] != "bar") {
	# I am not happy with the value of foo; this is an error
	header("HTTP/1.1 400 Invalid Request");
	die("An HTTP error 400 (invalid request) occurred.");
}

if (!file_exists($input_file_path)) {
	header("HTTP/1.1 404 File Not Found");
	die("HTTP error 404 occurred: File not found ($input_file_path)");
}

• header can also be used to send back HTTP error codes
  • header("HTTP/1.1 403 Forbidden");
  • header("HTTP/1.1 404 File Not Found");
  • header("HTTP/1.1 500 Server Error");

Checking for a mandatory query parameter

function get_query_param($name) {
	if (!isset($_GET[$name])) {
		header("HTTP/1.1 400 Invalid Request");
		die("HTTP/1.1 400 Invalid Request: missing required parameter '$name'");
	}
	if ($_GET[$name] == "") {
		header("HTTP/1.1 400 Invalid Request");
		die("HTTP/1.1 400 Invalid Request: parameter '$name' must be non-empty");
	}
	return $_GET[$name];
}

The $_SERVER superglobal array

• call phpinfo(); to see a complete list

GET or POST?

if ($_SERVER["REQUEST_METHOD"] == "GET") {
	# process a GET request
	...
} elseif ($_SERVER["REQUEST_METHOD"] == "POST") {
	# process a POST request
	...
}

• some web services process both GET and POST requests
• to find out which kind of request we are currently processing,
  look at the global $_SERVER array's "REQUEST_METHOD" element

Emitting partial-page HTML data

# suppose my web service accepts a "type" query parameter ...
<?php if ($_GET["type"] == "html") { ?>
	<ul>
		<?php foreach ($students as $kid) { ?>
			<li> <?= $kid ?> </li>
		<?php } ?>
	</ul>
<?php } ?>

• some web services do output HTML, but not a complete page
• the partial-page HTML is meant to be fetched by Ajax and injected into an existing page

Exercise: Baby name web service XML

• Modify our babynames.php service to produce its output as XML. For the data:

  Morgan m 375 410 392 478 579 507 636 499 446 291 278 332 518
  

• The service should output the following XML:

  <?xml version="1.0" encoding="UTF-8"?>
  <baby name="Morgan" gender="m">
  	<rank year="1890">375</rank>
  	<rank year="1900">410</rank>
  	...
  	<rank year="2010">518</rank>
  </baby>
  

Emitting XML data manually

...
header("Content-type: text/xml");
print "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n";
print "<books>\n";
foreach ($books as $book) {
	print "  <book title=\"{$book['title']}\" author=\"{$book['author']}\" />\n";
}
print "</books>\n";

• specify a content type of text/xml or application/xml
• print an XML prologue (the <?xml line), then print XML data as output
  • important: no whitespace output can precede the prologue; must be printed
• messy; bad to embed XML syntax in prints; write-only (hard to read existing XML data)

PHP's XML DOM: DOMDocument

The PHP DOMDocument class represents an XML document. It has these methods:

PHP's XML DOM: DOMElement

The PHP DOMElement class represents each DOM element. It has these fields/methods:

PHP XML DOM example

...
$xmldoc = new DOMDocument();                          # <?xml version="1.0"?>
$books_tag = $xmldoc->createElement("books");
$xmldoc->appendChild($books_tag);                     # <books>
foreach ($books as $book) {                                  
	$book_tag = $xmldoc->createElement("book");         #   <book
	$book_tag->setAttribute("title", $book["title"]);   #    title="Harry Potter" />
	$book_tag->setAttribute("author", $book["author"]); #    author="J.K. Rowling" />
	$books_tag->appendChild($book_tag);                      
}                                                     # </books>
header("Content-type: text/xml");
print $xmldoc->saveXML();

• much easier to read/write/manipulate complex XML
• saveXML automatically inserts the XML prolog for us

Exercise solution: Baby name web service XML

# takes a line of rankings and produces XML in the specified format
# example:  Aaron m 147 193 187 199 250 237 230 178 52 34 34 41 55
function generate_xml($line, $name, $gender) {
	$xmldom = new DOMDocument();
	$baby_tag = $xmldom->createElement("baby");     # <baby>
	$baby_tag->setAttribute("name", $name);
	$baby_tag->setAttribute("gender", $gender);
	
	$year = 1890;
	$tokens = explode(" ", $line);
	for ($i = 2; $i < count($tokens); $i++) {
		$rank_tag = $xmldom->createElement("rank");   # <rank>
		$rank_tag->setAttribute("year", $year);
		$rank_tag->appendChild($xmldom->createTextNode($tokens[$i]));
		$baby_tag->appendChild($rank_tag);
		$year += 10;
	}
	
	$xmldom->appendChild($baby_tag);
	return $xmldom;
}

Exercise: Baby name web service JSON

• Modify our babynames.php service to produce its output as JSON. For the data:

  Morgan m 375 410 392 478 579 507 636 499 446 291 278 332 518
  

• The service should output the following JSON:

  {
  	name: "Morgan",
  	gender: "m",
  	rankings: [375, 410, 392, 478, 579, 507, 636, 499, 446, 291, 278, 332, 518]
  }
  

Emitting JSON data manually

...
header("Content-type: application/json");
print "{\n";
print "  \"books\": [\n";
foreach ($books as $book) {
	print "  {\"title\": \"$title\"}\n";
}
print "\n";

• specify a content type of text/xml or application/xml
• print an XML prologue (the <?xml line), then print XML data as output
  • important: no whitespace output can precede the prologue; must be printed
• messy; bad to embed XML syntax in prints; write-only (hard to read existing XML data)

PHP's JSON functions

PHP includes the following global functions for interacting with JSON data:

• json_encode will output associative arrays as objects and normal arrays as arrays

PHP JSON example

<?php
$data = array(
	"library" => "Odegaard",
	"category" => "fantasy",
	"year" => 2012,
	"books" => array(
		array("title" => "Harry Potter", "author" => "J.K. Rowling"),
		array("title" => "The Hobbit", "author" => "J.R.R. Tolkien"),
		array("title" => "Game of Thrones", "author" => "George R. R. Martin"),
		array("title" => "Dragons of Krynn", "author" => "Margaret Weis"),
	)
);

header("Content-type: application/json");
print json_encode($data);
?>

PHP JSON example - output

{
	"library": "Odegaard",
	"category": "fantasy",
	"year": 2012,
	"books": [
		{"title": "Harry Potter", "author": "J.K. Rowling"},
		{"title": "The Hobbit", "author": "J.R.R. Tolkien"},
		{"title": "Game of Thrones", "author": "George R. R. Martin"},
		{"title": "Dragons of Krynn", "author": "Margaret Weis"},
	]
}