Introduction to Database and Web Application Programming ...bretscher/b20/lectures/w8.pdf · • LAMP (Linux, Apache, MySQL, PHP) is a common Web application platform – all components

CSCB20 – Week 8

Introduction to Database and Web Application Programming

Anna Bretscher*

Winter 2016 *thanks to Alan Rosselet for providing the slides these are adapted from.

Web Programming We have seen how HTML and CSS work together to create a Web page (HTML) with styling details applied (CSS) When you type a URL for an HTML document into a browser window:

o  browser sends a request to the server (mathlab in this case), o  the server locates the requested file (test.html), o  sends that file back to the browser to be rendered

Rather than providing the name of a file in a URL, it is possible to give the name of a program:

o  server executes the program o  sends its output back to the browser to be rendered, e.g.: https://mathlab.utsc.utoronto.ca/…/hello.php

What is PHP? PHP == ‘PHP Hypertext Preprocessor’.

o  Free and open-source, server-side scripting language designed specifically for the Web

o  Used to generate dynamic web-pages

o  Supported by most Web servers

PHP scripts o  are bracketed by reserved PHP tags

o  supports embedding of PHP scripts within HTML pages

o  easy to learn operational behavior and common patterns for working with Web pages

PHP Overview (cont’d) •  Interpreted language

•  Scripts are parsed at run-time rather than compiled beforehand

•  Executed on the server-side

•  Source-code not visible to client

•  ‘View Source’ in browsers does not display PHP code, only output produced by PHP code

•  Various built-in functions allow for fast development

•  Compatible with many popular databases

•  LAMP (Linux, Apache, MySQL, PHP) is a common Web application platform – all components are free, open-source

•  Syntax Perl- and C-like syntax. Relatively easy to learn

•  Large function library

•  Embedded directly into HTML

•  Interpreted, no need to compile

•  Loosely typed, like Python and JavaScript

PHP CSCB20 Databases and Web Apps

5

PHP Overview

PHP is but one of many server-side languages for developing dynamic Web app’s, other options include:

o  Java Servlets with JSP, Ruby on Rails, ASP .Net

Why choose PHP?

o  easy deployment – no complex infrastructure to set up

o  compatible: supported by most popular Web servers

o  simple: lots of built-in functionality; familiar syntax

o  free and open source: anyone can run a PHP-enabled server free of charge

o  available: installed on most commercial Web hosts, and on UTSC servers, including mathlab

Why PHP?

browser requests .html file (static content); o  server sends file content

browser requests .php file (dynamic content); o  server reads file, o  executes any embedded script content, o  sends output of script back to browser.

PHP Web Page Request Lifecycle

What does PHP code look like?•  Structurally similar to C/C++, Java •  Supports procedural and object-‐oriented paradigms •  All PHP statements end with a semi-‐colon •  Each PHP script must be enclosed in the reserved PHP

tag, denoted by

•  PHP code block may contain statements, funcJon definiJons, variable-‐value references

<?php … ?>

<!-– hello.php --> <html><body> <strong>Hello World!</strong><br /> <?php print "<h2>Hello, World</h2>"; ?> </body></html>

Hello World in PHP

Output generated by PHP “print” and “echo” statements is inserted into the HTML returned to the browser. Q. How do you view the output of a PHP script from a browser? Place hello.php in your cscb20w16_space directory, then view in a browser with URL: https://mathlab.utsc.utoronto.ca/courses/cscb20w16/UTORid/hello.php

Variables in PHP •  PHP variables begin with a “$” sign, both for

declaration and value reference

•  Case-sensitive ($Foo != $foo != $fOo)

•  Global and locally-scoped variables o  global variables can be used anywhere o  local variables restricted to a function or class

•  Certain variable names reserved by PHP o  Form variables ($_POST, $_GET) o  Server variables ($_SERVER) o  Etc.

Variable Usage and Comments

<?php $foo = 25; // Numerical variable $bar = “Hello”; // String variable $foo = ($foo * 7); // Multiplies foo by 7 $bar = ($bar * 7); // Invalid expression ?>

single-line comments are written as one of:

// single-line comment

# single-line comment multi-line comments bracketed by

/* multi-line comment ... */

Data TypesPHP is a loosely-typed language, like Python PHP basic types are:

o  int, float, boolean, string, array, object, NULL o  functions is_type() test whether a variable has a certain

type, e.g. is_string($myvar) Conversion between types is automatic in many cases, e.g.:

o  string to int for “+” o  int to float for “/”

Types can be “cast” to another type using:

$int_val = (int) “33”;

Strings $myvar = "hello"; print $myvar[1]; # prints “e”

square bracket notation for 0-based indexing catenation using “.” operator (not “+”)

print $myvar . "world"; # prints hello world strings quoted with double quotes are “interpreted”, meaning that embedded variables have their values inserted strings quoted with single quotes are not interpreted

print "$myvar world"; # prints hello world print '$myvar world'; # prints $myvar world

for loop for (initialization; condition; update) { statements

} uses same syntax as Java

for ($i = 10; $i >= 0; $i--) {

print "$i cubed is " . $i * $i * $i . ".\n";

}

for loop for (initialization; condition; update) { statements

} uses same syntax as Java

$name = "preprocessor";

for ($i = 0; $i < strlen($name); $i++) {

print "The next letter is".{$name[$i]}\n"; }

if/else statement if (condition) { statements;

} elseif (condition) { statements;

} else {

statements; }

•  elseif clause and else clause are both optional •  multiple elseif clauses may be used

<?php if ($user==“John”) { print “Hello John.”; } else { print “You aren’t John.”; } ?>

while loop same syntax as Java

while (condition) { statements;

}

or

do {

statements; } while (condition)

<?php $count=0; while($count<3) {

print “hello PHP. ”; $count += 1; // or // $count = $count + 1; // or // $count++;

} ?>

hello PHP. hello PHP. hello PHP.

Arrays (ie, lists) $myvar = array(); # create new array

$myvar = array(val0, val1, ..., valN);

$myvar[index]; # element at position index

$myvar[index] = val0; # assign element at index

$myvar[] = valN; # append valN

$a1 = array(); # empty, length-0 array

$a[2] = 12; # store 12 in 3rd position of array

$a2 = array("a", "sequence", "of", "strings"); $a2[] = "the end"; # new last element of $a2

foreach loop foreach ($array as $element) {

... }

Similar to Pythonʼ’s: for element in array: Simpler than regular “for” loop when using arrays $a = array("a", "sequence", "of", "strings"); for ($i = 0; i < count($a); $i++) {

print "the next word is {$a[$i]}\n"; } foreach ($a as $element) {

print "the next word is $element\n"; }

Embedded PHP We could use PHP print and/or echo statements to generate HTML output, e.g. <?php print "<html>\n<head>\n"; print "<title>PHP Squares</title>\n"; ... for ($i = 0; i <= 10; $i++) { print "<p>$i squared is $i*$i</p>\n";

} ?> What’s wrong with this approach? Suppose you want to change the page HTML …

Embedding PHP in HTMLWrite HTML literally. When scripting is needed to compute a value, embed PHP code. General format of a PHP script written within HTML file:

HTML elements ...  <?php

PHP code ... # output embedded within HTML ?> HTML elements ... <?php

PHP code ... ?> HTML elements ...

Embedding PHP in HTMLGeneral format of a PHP script written within HTML file: HTML elements ...  <?php PHP code ... # output embedded within HTML

?> HTML elements ... The PHP code in an embedded block may consist of statements, declarations, or expression values. Here’s a sample expression block: <?= $myvar ?>

which is equivalent to

<?php print $myvar; ?>

Embedding PHP in HTMLHereʼ’s our earlier “squares” calculator, with “poor style” print statements:

<?php

print "<html>\n<head>\n";

print "<title>PHP Squares</title>\n"; ...

for ($i = 0; i <= 10; $i++) {

print "<p>$i squared is $i*$i</p>\n";

}

?>

Embedding PHP in HTMLHereʼ’s our earlier “squares” calculator, now without print statements:

<html><head>

<title>PHP Squares</title>

... <?php

for ($i = 0; $i <= 10; $i++) { ?>

<p><?= $i ?> squared is <?= $i*$i ?> </p>

<?php } ?>

... </html>

FunctionsFunctions must be defined before then can be called. Function headers are of the format:

Note that no return type is specified.

function quadratic($a, $b, $c) { return -$b + sqrt($b*$b - 4*$a*$c) / (2*$a); } $x = -2; $y = 3; $root = quadratic(1, $x, $y-2);

Unlike variables, function names are not case sensitive foo(…) == Foo(…) == FoO(…)

function functionName($arg_1, $arg_2, …, $arg_n)

Query Strings and Parameters•  We refer to Web pages using URL’s (Uniform Resource Locators),

of the form http://domain_name/path_value

•  We can specify parameters to PHP scripts by appending a value to the end of the URL:

http://www.google.com/search?q=androidhttps://mathlab…./cscb20w16/utorid/rancid.php?film=vampire

•  Parameter name=value pairs follow the “?” at the end of the URL path_value, in 2nd example param name is film, value is vampire

•  Provides a mechanism by which a user can control/customize the behavior of a server-side PHP script

Query Strings and Parameters•  PHP can retrieve parameter values using the $_REQUEST array:

$_REQUEST["parameter_name"]

•  Returns the parameter’s value as a string

•  Can check to see if a specific parameter is set using isset():

$country_name = $_REQUEST["country"]; $population = (int) $_REQUEST["population"]; if (isset($_REQUEST["code"])) {

$code = (int) $_REQUEST["code"]; } else { $code = -1;

}

Reading Files Two ways to read the contents of a text file: 1. file(“my_file.txt”); returns array of lines in my_file.txt 2. file_get_contents(“my_file.txt”); returns a single string containing all lines in my_file.txt <?php # display lines from file as a bulleted list $cities = file("cities.txt"); foreach ($cities as $city) { ?> <li> <?= $city ?> </li> <?php } ?>

Unpacking Arrays, Splitting StringsSometimes it is useful to be able to refer to the elements of an array by individual variable names, rather than using indices $movie_info = file("info.txt");

list($title,$year) = $movie_info;

# now can use $title rather than $movie_info[0]

A string consisting of delimited values can be split (same idea as in Python)

$title = "Databases and Web Programming";

$words = explode(" ", $title);

Reading Directories •  If your application needs to read from a set of files in a

directory, how can your code automatically detect and read the specific files present?

•  glob enables you to use pattern matching to select files

$notes = glob("note_*.txt");

foreach ($notes as $note) {

print $note;

}

•  * is just one of several “regular expression” pattern-matching forms (others include matching on character ranges, matching digits, optional characters)

Web Services •  In Assignment 2 we will use PHP embedded within an

HTML document to implement dynamic HTML content •  However, HTML is only one of several kinds of data a

server could produce for use by a client

•  A Web service refers to use of the Web’s HTTP protocol to invoke programs and retrieve their results

•  The general idea is that we should be able to call programs using URL references, just as we do to refer to Web pages

•  Like tradiJonal funcJons, Web-‐service programs can take parameters and produce results, which may be wriXen as HTML, but also as XML, JSON, plain text, or other formats

Web Services and PHP •  The type of output produced by a Web service must be

explicitly specified, since it can take different

•  The client needs to know how to interpret the byte values returned by the server

•  HTTP, the Internet protocol used for Web URL requests and responses, provides a “Content-type” header for this purpose

•  In PHP, the “type” of the result value(s) defaults to HTML (“text/html”), but can be explicitly specified using:

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

•  The header() function must be called before a PHP script generates any output (since the client who called the script needs the header information to interpret that output)

MIME Content-‐Types •  MIME types are used to communicate the type of data sent

by a server to a client (e.g. a jpeg image, or an html file), and vice versa (e.g. a file upload from a client)

•  MIME types are specified in two parts: “type/subtype”, e.g.:

MIME type related file extension text/plain .txt text/html .html, .htm, ... text/css .css application/json image/png .png image/jpg .jpeg, .jpg, .jpe text/javascript .js

A PHP Web service •  Let’s examine a simple example of a PHP Web service that

take “base” and “exp” parameters, and returns the base raised to the exp (exponent) power.

•  A URL to invoke this service might look like this:

hXps://mathlab…/cscb20w16/utorid/power.php?base=5&exp=3 •  How would we implement this service in PHP?

<?php header("Content-type: text/plain");

$base = (int) $_GET["base"];

$exp = (int) $_GET["exp"];

$result = pow($base, $exp);

print $result; ?>

Web Service Errors •  When implementing a Web service, we must make

provision for errors, such as omission of a required parameter or an invalid parameter value. E.g.

https://mathlab…/utorid/power.php?base=5&exp=w

https://mathlab…/utorid/power.php?base=5

•  How should such an error be reported?

•  We could return an HTML error message, but what if the client (caller) takes the result and displays it in a result <div> on their Web page, now they display an opaque error message where the user expects a number

•  We need a mechanism that will enable the caller to detect that the result is an error, as opposed to a result value.

HTTP Status Codes The Web’s HTTP protocol provides a mechanism for signaling the outcome of a request, that can be used for both ordinary Web pages (e.g. 404 Not Found), and for Web services (e.g. 400 illegal request)

HTTP code

Meaning

200 OK 301-303 page has moved (temporarily or

permanently) 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

A Web Service with Error HandlingWe could rewrite the power() Web service to detect missing or invalid parameters as follows:

<?php $base = $_GET["base"]; $exp = $_GET["exp"]; if (is_numeric($base) and is_numeric($exp)) { header("Content-type: text/plain"); ... as before for valid input ... } else {

header("HTTP/1.1 400 Invalid Request"); die("invalid request; required parameters"); } ?>

Web Service Output Types So far, our Web service examples have output values expressed as MIME type text/plain.

More commonly, a Web service invoked from a Web page will return an HTML fragment, XML data, or JSON data.

Why an HTML fragment? Because normally the result returned by a Web service will be inserted into an existing HTML document, e.g. as the content of a <div>

Web Service Output Types Suppose we want to generate an HTML list of factorial values, up to a user-supplied value of “n”: <?php header("Content-type: text/html"); $limit = (int) $_GET["n"]; $fact = 1; for ($i = 1; $i < $limit; $i++) { ?> <li>Factorial of <?= $i ?> is <?= $fact ?> </

li> <?php $fact = $fact * $i; } ?>

Later we’ll look at how an HTML fragment, like the one generated by this script could be inserted into a Web page

Documents

Introduction to Database and Web Application Programming ...bretscher/b20/lectures/w8.pdf · • LAMP (Linux, Apache, MySQL, PHP) is a common Web application platform – all components