Abstract classes
A class may be declared abstract to prevent it from being instantiated.
However, you may inherit from an abstract class:
abstract class MyBaseClass {
function display() {
print “Default display routine being called”;
}
}
Abstract methods.
A method may be declared abstract, thereby deferring its definition to an
inheriting class. A class that includes abstract methods must be declared
abstract:
abstract class MyBaseClass {
abstract function display();
}
Class type hints.
Function declarations may include class type hints for their parameters.
If the functions are called with an incorrect class type, an error occurs:
function expectsMyClass(MyClass $obj) {
}
Support for dereferencing objects that are returned from methods.
In PHP 4, you could not directly dereference objects that were returned
from methods. You had to first assign the object to a dummy variable and
then dereference it.
PHP 4:
$dummy = $obj->method();
$dummy->method2();
PHP 5:
$obj->method()->method2();
Iterators.
PHP 5 allows both PHP classes and PHP extension classes to implement
an Iterator interface. After you implement this interface, you can iterate
instances of the class by using the foreach() language
construct:
$obj = new MyIteratorImplementation();
foreach ($obj as $value) {
print “$value”;
}
1.2 Language Features
For a more complete example, see Chapter 4, “PHP 5 Advanced OOP and
Design Patterns.”
__autoload().
Many developers writing object-oriented applications create one PHP
source file per class definition. One of the biggest annoyances is having to
write a long list of needed inclusions at the beginning of each script (one for
each class). In PHP 5, this is no longer necessary. You may define an
__autoload() function that is automatically called in case you are trying to use
a class that has not been defined yet. By calling this function, the scripting
engine offers one last chance to load the class before PHP bails out with an
error:
function __autoload($class_name) {
include_once($class_name . “php”);
}
$obj = new MyClass1();
$obj2 = new MyClass2();
1.2.3 Other New Language Features
Exception handling.
PHP 5 adds the ability for the well-known try/throw/catch structured
exception-handling paradigm. You are only allowed to throw objects that
inherit from the Exception class:
class SQLException extends Exception {
public $problem;
function __construct($problem) {
$this->problem = $problem;
}
}
try {
…
throw new SQLException(”Couldn’t connect to database”);
…
} catch (SQLException $e) {
print “Caught an SQLException with problem $obj->problem”;
} catch (Exception $e) {
print “Caught unrecognized exception”;
}
Currently for backward-compatibility purposes, most internal functions
do not throw exceptions. However, new extensions make use of this capability,
and you can use it in your own source code. Also, similar to the already exist-
ing set_error_handler(), you may use set_exception_handler() to catch an
unhandled exception before the script terminates.
foreach with references.
In PHP 4, you could not iterate through an array and modify its values.
PHP 5 supports this by enabling you to mark the foreach() loop with the
& (reference) sign, which makes any values you change affect the array
over which you are iterating:
foreach ($array as &$value) {
if ($value === “NULL”) {
$value = NULL;
}
}
Default values for by-reference parameters.
In PHP 4, default values could be given only to parameters, which are
passed by-values. PHP 5 now supports giving default values to by-
reference parameters:
function my_func(&$arg = null) {
if ($arg === NULL) {
print ‘$arg is empty’;
}
}
my_func();
1.3 GENERAL PHP CHANGES
1.3.1 XML and Web Services
Following the changes in the language, the XML updates in PHP 5 are proba-
bly the most significant and exciting. The enhanced XML functionality in PHP
5 puts it on par with other web technologies in some areas and overtakes them
in others.
1.3.1.1 The Foundation
XML support in PHP 4 was implemented using a
variety of underlying XML libraries. SAX support was implemented using the
old Expat library, XSLT was implemented using the Sablotron library (or using
libxml2 via the DOM extension), and DOM was implemented using the more
powerful libxml2 library by the GNOME project.
Using a variety of libraries did not make PHP 4 excel when it came to
XML support. Maintenance was poor, new XML standards were not always
supported, performance was not as good as it could have been, and interopera-
bility between the various XML extensions did not exist.
In PHP 5, all XML extensions have been rewritten to use the superb
libxml2 XML toolkit (http://www.xmlsoft.org/). It is a feature-rich, highly main-
tained, and efficient implementation of the XML standards that brings cutting-
edge XML technology to PHP.
1.3 General PHP Changes
All the afore-mentioned extensions (SAX, DOM, and XSLT) now use
libxml2, including the new additional extensions SimpleXML and SOAP.
1.3.1.2 SAX
As previously mentioned, the new SAX implementation has
switched from using Expat to libxml2. Although the new extension should be
compatible, some small subtle differences might exist. Developers who still
want to work with the Expat library can do so by configuring and building
PHP accordingly (which is not recommended).
1.3.1.3 DOM
Although DOM support in PHP 4 was also based on the libxml2
library, it had bugs, memory leaks, and in many cases, the API was not W3C-
compliant. The DOM extension went through a thorough facelift for PHP 5. Not
only was the extension mostly rewritten, but now, it is also W3C-compliant. For
example, function names now use studlyCaps as described by the W3C standard,
which makes it easier to read general W3C documentation and implement what
you have learned right away in PHP. In addition, the DOM extension now sup-
ports three kinds of schemas for XML validation: DTD, XML schema, and
RelaxNG.
As a result of these changes, PHP 4 code using DOM will not always run
in PHP 5. However, in most cases, adjusting the function names to the new
standard will probably do the trick.
1.3.1.4 XSLT
In PHP 4, two extensions supported XSL Transformations: the
Sablotron extension and the XSLT support in the DOM extension. PHP 5 fea-
tures a new XSL extension and, as previously mentioned, it is based on the
libxml2 extension. As in PHP 5, the XSL Transformation does not take the
XSLT stylesheet as a parameter, but depends on the DOM extension to load it.
The stylesheet can be cached in memory and may be applied to many docu-
ments, which saves execution time.
1.3.1.5 SimpleXML
When looking back in a year or two, it will be clear that
SimpleXML revolutionized the way PHP developers work with XML files.
Instead of having to deal with DOM or–even worse–SAX, SimpleXML repre-
sents your XML file as a native PHP object. You can read, write, or iterate over
your XML file with ease, accessing elements and attributes.
Consider the following XML file:
<clients>
<client>
<name>John Doe</name>
<account_number>87234838</account_number>
</client>
<client>
<name>Janet Smith</name>
<account_number>72384329</account_number>
</client>
</clients>
The following code prints each client’s name and account number:
$clients = simplexml_load_file(’clients.xml’);
foreach ($clients->client as $client) {
print “$client->name has account number $client
>account_number\n”;
}
It is obvious how simple SimpleXML really is.
In case you need to implement an advanced technique in your Sim-
pleXML object that is not supported in this lightweight extension, you can
convert it to a DOM tree by calling it dom_import_simplexml(), manipulate it in
DOM, and convert it to SimpleXML using simplexml_import_dom().
Thanks to both extensions using the same underlying XML library,
switching between them is now a reality.
1.3.1.6 SOAP
PHP 4 lacked official native SOAP support. The most com-
monly used SOAP implementation was PEARs, but because it was imple-
mented entirely in PHP, it could not perform as well as a built-in C extension.
Other available C extensions never reached stability and wide adoption and,
therefore, were not included in the main PHP 5 distribution.
SOAP support in PHP 5 was completely rewritten as a C extension and,
although it was only completed at a very late stage in the beta process, it was
incorporated into the default distribution because of its thorough implementa-
tion of most of the SOAP standard.
The following calls SomeFunction() defined in a WSDL file:
$client = new SoapClient(”some.wsdl”);
$client->SomeFunction($a, $b, $c);
1.3.1.7 New MySQLi (MySQL Improved) Extension
For PHP 5, MySQL AB
(http://www.mysql.com) has written a new MySQL extension that enables you
to take full advantage of the new functionality in MySQL 4.1 and later. As
opposed to the old MySQL extension, the new one gives you both a functional
and an OO interface so that you can choose what you prefer. New features sup-
ported by this extension include prepared statements and variable binding,
SSL and compressed connections, transaction control, replication support, and
more.
1.3.1.8 SQLite Extension
Support for SQLite (http://www.sqlite.org) was
first introduced in the PHP 4.3.x series. It is an embedded SQL library that
does not require an SQL server, so it is suitable for applications that do not
require the scalability of SQL servers or, if you deploy at an ISP that does not offer access to an SQL server. Contrary to what its name implies, SQLite has
many features and supports transactions, sub-selects, views, and large data-
base files. It is mentioned here as a PHP 5 feature because it was introduced
so late in the PHP 4 series, and because it takes advantage of PHP 5 by pro-
viding an OO interface and supporting iterators.
1.3.1.9 Tidy Extension
PHP 5 includes support for the useful Tidy (http://
tidy.sf.net/) library. It enables PHP developers to parse, diagnose, clean, and
repair HTML documents. The Tidy extension supports both a functional and
an OO interface, and its API uses the PHP 5 exception mechanism.
1.3.1.10 Perl Extension
Although not bundled in the default PHP 5 package,
the Perl extension allows you to call Perl scripts, use Perl objects, and use
other Perl functionality natively from within PHP. This new extension sits
within the PECL (PHP Extension Community Library) repository at http://
pecl.php.net/package/perl.
1.4 OTHER NEW FEATURES IN PHP 5
This section discusses new features introduced in PHP 5.
1.4.1 New Memory Manager
The Zend Engine features a new memory manager. The two main advantages
are better support for multi-threaded environments (allocations do not need to
perform any mutual exclusion locks), and after each request, freeing the allo-
cated memory blocks is more efficient. Because this is an underlying infra-
structure change, you will not notice it directly as the end user.
1.4.2 Dropped Support for Windows 95
Running PHP on the Windows 95 platform is not supported anymore due to
Windows 95 does not support the functionality that PHP uses. Because
Microsoft officially stopped supporting it in 2002, the PHP development com-
munity decided that dropping the support was a wise decision.
1.5 SUMMARY
You must surely be impressed by the amount of improvements in PHP 5. As
previously mentioned, this chapter does not cover all the improvements, but
only the main ones. Other improvements include additional features, many
bug fixes, and a much-improved infrastructure. The following chapters cover
PHP 5 and give you in-depth coverage of the named new features and others
that were not mentioned in this chapter.