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232
StreamsandNetworkProgramming
is
_
readable()
—Checks ifthe path existsand isreadable
is
_
writable()
—Checks ifthe path existsand iswritable
is
_
uploaded
_
file()
—Checks if the path is an uploaded file (sent via HTTP
POST)
Each ofthese functions returns a Boolean value; notethat the results of a call to any
of these functions will be cached, so that two calls to a given function on the same
stream resource and during the same script will return the same value, regardless
of whether the underlying resource has changed in the meantime. Given the rela-
tively short lifespanof a script, this is not generally a problem—but it is something
to keep in mind whendealing with long-running scripts, or with scripts whose pur-
pose is precisely that ofwaiting for a resource to change. For example, consider the
following script:
$f = ’/test/file.txt’;
while (!is
_
readable($f)) {}
$data = file
_
get
_
contents();
Besides the obviously unhealthy practice of performing an operationinside an infi-
nite loop, this code has the added handicap that, if
/test/file.txt
is not readable
whenthe script first enters intothe
while()
loop,this script will never stop running,
evenif the file later becomes readable, since the data is cached when
is
_
readable()
is first executed.
i
TheinternalcachemaintainedwithinPHPforthesefunctionscanbeclearedbycalling
clearstatcache()
.
File permissions on UNIX systems can be changed using a number of functions,
including
chmod()
,
chgrp()
and
chown()
.For example:
chmod (’/test/file.txt’, 0666);
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StreamsandNetworkProgramming
233
Note how
chmod()
inparticular takes a numeric value for the file’s permissions—text
permissions specifiers like
gu+w
are not allowed. As you can see above, the octal no-
tation makes it easier to use the same values that you would use when calling the
chmod
UNIX shell utility.
Accessing Network Resources
As we mentioned earlier, one of the strongest points of the streams layer is the fact
that the same set of functionality that you use to access files can be used to access
anumber ofnetwork resources, oftenwithoutthe need for anyspecial adjustments.
This has the great advantage ofboth greatly simplifying tasks like opening a remote
Web page, or connecting to an FTP server, while at the same time also eliminating
theneed to learnanother setoffunctions.
Simple Network Access
The easiest way to access a network resource consists of treating it in exactly the
same way as a file. For example, suppose you wanted to load up the main page of
php|architect:
$f = fopen (’http://www.phparch.com’);
$page = ’’;
if ($f) {
while ($s = fread ($f, 1000)) {
$page .= $s;
}
} else {
throw new Exception ("Unable to open connection to www.phparch.com");
}
Clearly, not all file functions may work with a given network resource; for example,
you cannot write to an HTTP connection, because doing so is not allowed by the
protocol, and would not make sense.
One aspect ofstreams thatis not always immediately obvious is the fact that they
affect pretty much all of PHP’s file access functionality—including
require()
and
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234
StreamsandNetworkProgramming
include()
;for example, the following is perfectly valid (depending onyour configu-
ration):
include ’http://phparch.com’;
This capability is, of course, something that you should both love and fear: on one
hand,itallows you include remote filesfroma differentserver. Onthe other,it repre-
sents a potential security hole ofmonumental proportions ifthe wrong persongets
their handsonyour code.
Stream Contexts
Streamcontexts allow you to pass options to the streamhandlers that you transpar-
ently use to access network resources, thus allowing you to tweak a handler’s be-
haviour in ways that go beyond what normal file functionality can do. For example,
you can instruct the HTTP stream handler to perform a POST operation—which is
very handywhenyou want to work with Web services.
Streamcontextsare created using
stream
_
create
_
context()
:
$http
_
options = stream
_
context
_
create(array(
’http’ => array(
’user
_
agent’ => "Davey Shafiks Browser",
’max
_
redirects’ => 3
)
));
$file = file
_
get
_
contents("http://localhost/", false, $http
_
options);
In this example, we set context options for the
http
stream, providing our own cus-
tom user agent string (which is always the polite thing to do to help people identify
the activity you perform on their server),and set themaximumnumber of transpar-
ent redirections to three. Finally, as you can see, we pass the newly-created context
asa parameter to
file
_
get
_
contents()
.
Advanced Stream Functionality
While the built-in stream handlers cover the most common network and file op-
erations, there are some instances—such as when dealing with custom proto-
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StreamsandNetworkProgramming
235
cols—whenyou need to take matters into your own hands. Luckily,the stream layer
makes eventhismuch easier to handle than,say, ifyou wereusingC. Infact,you can
create socketservers and clients using the streamfunctions
stream
_
socket
_
server()
and
stream
_
socket
_
client()
,and thenuse the traditional file functions to exchange
information:
$socket = stream
_
socket
_
server("tcp://0.0.0.0:1037");
while ($conn = stream
_
socket
_
accept($socket)) {
fwrite($conn, "Hello World\n");
fclose($conn);
}
fclose($socket);
We can
then
connect to
this simple “Hello
World” server using
stream
_
socket
_
client()
.
$socket = stream
_
socket
_
client(’tcp://0.0.0.0:1037’);
while (!feof($socket)) {
echo fread($socket, 100);
}
fclose($socket);
Finally,wecanrunour server justlike any other PHP script:
$ php ./server.php &
and our client:
$ php ./client.php
Hello World
Stream Filters
Stream filters allow you to pass data in and out of a stream through a series of fil-
ters that can alter it dynamically, for example changing it to uppercase, passing it
through a ROT-13 encoder, or compressing it using
bzip2
.Filters ona given stream
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236
StreamsandNetworkProgramming
are organized ina chain—thus,you cansetthem up sothat the data passes through
multiple filters, sequentially.
You can add a filter to a stream by using
stream
_
filter
_
prepend()
and
stream
_
filter
_
append()
—which, as you might guess, add a filter to the beginning
and end ofthefilter chainrespectively:
$socket = stream
_
socket
_
server("tcp://0.0.0.0:1037");
while ($conn = stream
_
socket
_
accept($socket)) {
stream
_
filter
_
append($conn, ’string.toupper’);
stream
_
filter
_
append($conn, ’zlib.deflate’);
fwrite($conn, "Hello World\n");
fclose($conn);
}
fclose($socket);
In this example, we apply the
string.toupper
filter to our server stream, which will
convert the data to upper case, followed by the
zlib.deflate
filter to compress it
whenever we write data to it.
We can then apply the
zlib.inflate
filter to the client, and complete the imple-
mentationofa compressed data streambetweenserver and client:
$socket = stream
_
socket
_
client(’tcp://0.0.0.0:1037’);
stream
_
filter
_
append($socket, ’zlib.inflate’);
while (!feof($socket)) {
echo fread($socket, 100);
}
fclose($socket);
If you consider how complex the implementation of a similar compression mech-
anism would have normally been, it’s clear that stream filters are a very powerful
feature.
Summary
As you cansee, streams penetrate to the deepest levels of PHP, from general file ac-
cesstoTCP andUDP sockets. Itis evenpossible tocreate your ownstreamprotocols
and filters, making this the ultimate interface for sending and receiving data with
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StreamsandNetworkProgramming
237
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Appendix A
Differences Between PHP 4 and 5
Someofthe questionsintheexamtestyour understanding ofhowPHP5 differs from
previous versions. As such, it’s a good idea to be fully aware of at least the major
changes that have occurred betweenthe two versions.
Almost all theinformationcontainedinthisappendixhas already beencovered in
the preceding chapters; therefore, we present it here mostly for the sake of conve-
nience, and we do not dwell much on explanations—for more information on any
particular topic,you canrefer back to the appropriate section of this book, or tothe
PHP manual.
Language Features
• PHP 5 allows limited type hinting. This allows you to specify that the param-
eter to a function or class method can only be of a specific class (or one of its
subclasses),or anarray. However, you maynot specifyanyother scalar types.
• The
foreach
constructnowsupports by-referencedeclarationofthe value ele-
ment.
• Anumber ofnewfunctions,particularlyfor stringandarraymanipulation,has
alsobeen added to the core platform.
240
DifferencesBetweenPHP4 and 5
Objects
• For all intents and purposes, all objects inPHP 5 arepassed by reference. This
means that assigning an object to a variable will not create a copy of the for-
mer, but simplycreates another reference to it.
• Constants,aswellas static methods andproperties,cannowbe defined within
the scope of a class.
• Class methods and properties now feature visibility, and can be declared as
public
,
private
or
protected
. Classes and methods can also be declared as
final
to preventfurther inheritance.
• Sinceallobjectsare assignedbyreference,younowneed aspecializedmecha-
nismto copyobjects. This isprovided by the
clone
constructandthe__
clone()
magic method.
• PHP 5 features unified constructors and destructors—all constructors should
now be named__
construct()
,and the new__
destruct()
magic method has
beenadded for object destruction.
• With the addition ofinterfaces and abstract classes, PHP developers now have
far greater control over how they implement their object-oriented code. In-
terfaces can be used to define common APIs, while abstract classes provide
models for class implementations thatfollowa specific blueprint.
• Class definitions can now be loaded on demand by using the__
autoload()
function.
MagicMethods
Amultitude ofnew “magic” methods has beenintroduced in PHP 5:
• __
get()
and__
set()
are called when accessing or assigning anundefined ob-
ject property, while__
call()
is executed when calling a non-existent method
of a class.
DifferencesBetweenPHP4 and5
241
• __
isset()
is called when passing an undefined property to the
isset()
con-
struct.
• __
unset()
is called when passing anundefined property to
unset()
.
• __
toString()
is called whentrying to directly
echo
or
print()
an object.
• __
set
_
state()
is inserted dynamically by
var
_
export()
to allow for re-
initializationonexecutionof
var
_
export()
’s output.
Selected New Extensions
• SimpleXML allows easy access toXML data using objectand array notation.
• PHP 5 also introduces a DOMXML, DOMXSL and Sablotron replacement in
the formofthe libxml2-based DOM and XSLextensions.
• ThePHP Data Objects (PDO) extensionprovides a unified database accessex-
tension that allows access to many different types ofdatabase systems by us-
ing a commoninterface. PDO isnot anabstractionlayer—except for prepared
queries,it doesnothing to abstractthe actual databasecode (SQL), itself.
• Thehashextensionis anewreplacementfor theGPLed libmhash;itwasadded
to the PHP core starting with version5.1.2. Itcanproduce hashes using many
algorithms,including thefamiliar MD5andSHA1,aswellassomemore secure
(albeit slower) algorithms, such assnefru.
• The Standard PHP Library (SPL) provides numerous interfaces that enhance
the way classes interact with the PHP language, including the new
Iterator
interfaces.
• The new Reflection extension allows for runtime introspection of executing
PHP code.
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