PHP is a widely-used open-source programming language primarily for server-side applications and developing dynamic web content, and more recently, other software. The name is a
recursive acronym for "PHP: Hypertext Preprocessor". (This is
actually a retronym; see history.)
Famous examples of PHP applications include phpBB and MediaWiki, the software behind Wikipedia. The PHP model can be
seen as an alternative to Microsoft's ASP.NET/C#/VB.NET system, Macromedia's ColdFusion system, Sun Microsystems' JSP/Java system, and to the CGI/Perl system.
Overview
PHP's ease of use and similarity with the most common structured programming languages—most notably C and Perl (and from version 5, Java)—allows most
experienced programmers to start developing complex applications with a minimal
learning curve. It also enables experienced developers to get involved with dynamic web content applications
without having to learn a whole new set of functions and practices.
One of the more attractive parts of PHP is that it is more than just a scripting language. Due to its modular design, PHP is also used to develop GUI applications (using PHP-GTK), and can be used from the command line
just like Perl or Python.
PHP allows interaction with a large number of relational database management systems, such as MySQL, Oracle, IBM DB2, Microsoft SQL Server, PostgreSQL and SQLite while maintaining a
simple and straightforward syntax. PHP runs on most major operating
systems, including UNIX, Linux, Windows, and Mac OS X, and
can interact with many major web servers. The official PHP website (http://www.php.net/) contains extensive documentation (http://www.php.net/manual/). The Linux, Apache, MySQL, PHP
(LAMP) architecture has become popular in the Web industry as a way of deploying
inexpensive, reliable, scalable, secure web applications. (The 'P' in LAMP can also stand for Perl or Python.)
PHP is the result of the collective efforts of many contributors. It is licensed under a BSD-style license, the PHP license. PHP, from version 4,
has been powered by the Zend engine.
History
PHP was originally designed as a small set of Perl scripts, followed by a rewritten set of CGI binaries written in C by
Rasmus Lerdorf in 1994 to display
his résumé and collect some data, such as how many hits it was generating. Others first used "Personal Home
Page Tools" in 1995, when Lerdorf had combined it with his own Form Interpreter to
create PHP/FI. Zeev Suraski and Andi Gutmans, two Israeli developers of the Technion - Israel Institute of Technology, rewrote the parser in 1997
and formed the base of PHP 3. They also changed the name to its current recursive form. After months in beta, the development team officially released PHP/FI 2 in November 1997. Public testing of PHP 3 began
immediately and the official launch came in June 1998. Suraski and Gutmans then started a
new rewrite of PHP's core, producing the Zend engine in 1999 (a page at www.zend.com (http://www.zend.com/zend/zend-engine-summary.php) states that PHP 3 was powered by Zend
Engine 0.5). They also founded Zend Technologies in Ramat Gan, Israel which has since overseen the
PHP advances. In May 2000, PHP 4, powered by the Zend Engine 1.0, was released. On July 13, 2004, PHP 5 was released, powered by Zend
Engine II (formerly known as Zend Engine 2).
Popularity
PHP is currently one of the most popular server-side scripting systems on the Web. It has been widely
adopted since the release of version 4.
One major part of PHP which has helped it become popular is that it is a very loose language; in particular, it is dynamically typed. That is, the rules aren't as strict with
variables—they don't have to be declared and they can hold any type of object. Further, unlike many other languages (like
C++ and Java), arrays are able to hold objects of varying types, including
other arrays.
According to Netcraft's April 2002 survey,
PHP is now the most deployed server-side scripting language, running on around 9 of the 37 million domains in their survey. This
is confirmed by PHP's own figures, which show PHP usage (measured on a per-domain basis) growing at around 5% per month. In May
2003, almost 13 million domains were using PHP, based on the same source.[1] (http://www.php.net/usage.php)
Due to PHP's popularity, a new breed of programmer has emerged—one who is only familiar with PHP, which in turn forced
open the door toward a command line interface for PHP,
along with support for GUI library such as GTK+ and text mode libraries like Ncurses and Newt. This is a major step for PHP, because it represents its beginning adoption
as a genuine programming language (i.e. running autonomously on a stand-alone machine, as opposed to its original purpose of
serving web pages to client machines from a server).
Many PHP programmers have reported having had trouble trying to learn other languages in the past and ultimately giving up
after each attempt until attempting to learn PHP. The ease of programming in PHP has made it so these programmers are able to
learn the basics of programming and are then able to continue on to other languages suchs as C/C++ or Perl/Python/Java and then
finding themselves back programming in PHP for its speed of development in comparison to other languages.
Code example
Here is a Hello World code example: [2] (http://php.net/manual/en/language.basic-syntax.php)
<?php
echo "Hello, world!\n";
?>
Here is an example that prints out the lyrics for the song 99
Bottles of Beer:
<?php
/*
* /* ... */ is a comment that can span one or many lines.
* This kind of comment does not need stars (*) in the beginning of each line,
* but including them is a common practice. // and # are also comments.
* They only comment the text that are after them in the same line. They have
* no special ending character.
*
*/
/*
* First we define a new function called "plural".
* It will return an "s" if the argument passed to it was any other
* than number 1.
*/
function plural($number) {
return ($number != 1 ? "s" : "");
// The ternary ?: operator is similar to if-else: (test_condition ? true : false)
// In this case it's used to return "" for one and "s" for all other numbers
}
// We define a variable called $lb to contain an HTML line break
// as well as a carriage return and line feed:
$lb = "<br />\r\n";
for ($i = 99; $i > 0; $i--) {
echo $i . " bottle" . plural($i) . " of beer on the wall," . $lb;
// We don't actually need a new echo for each line. Let's see:
echo $i . " bottle" . plural($i) . " of beer." . $lb . "
Take one down, pass it around," . $lb .
($i - 1 != 0 ? $i - 1 : "no more") .
" bottle" . plural($i - 1) . " of beer on the wall" . $lb . $lb;
}
echo "Go to the store," . $lb . "buy some more," . $lb .
"99 bottles of beer on the wall!";
?>
Notes:
- PHP allows the placement of strings on multiple lines, as long as it eventually finds a semicolon (;) to terminate it.
[3]
(http://php.net/manual/en/language.basic-syntax.instruction-separation.php)
- A period (.) concatenates strings together. [4]
(http://php.net/manual/en/language.types.string.php)
- Variables always have names that start with a dollar sign ($), and are parsed inside double quotation marks ("), but not
inside single quotation marks ('). Functions, such as plural(), are never parsed inside any sort of string. [5]
(http://php.net/manual/en/language.variables.php)
- Although PHP allows both # and // for "same line" comments, it is generally preferred to use the C-style // and not the
Perl-style #. [6]
(http://us2.php.net/manual/en/language.basic-syntax.comments.php)
- For output, this program uses
echo. print and printf may also be used for this
purpose.
Libraries
PHP includes a large number of free and open-source libraries with the core build. PHP is a fundamentally Internet-aware system with modules built in for accessing FTP servers, many database servers, embedded SQL libraries like
embedded MySQL and SQLite, LDAP servers, and others. Many
functions familiar to C programmers such as the
printf family are available in the standard PHP build.
PHP extensions exist which, among other features,
add support for the Windows API, process management on UNIX-like operating
systems, cURL, and the ZIP/gzip/bzip2/rar/lzf compression formats. Some of the more unusual features
are on-the-fly Macromedia Flash generation, integration with
Internet relay chat, and generation of dynamic images (where
the content of the image can be changed). Some additional extensions are available via the PHP Extension Community Library (PECL).
This is the present list of all officially documented libraries:
(Source: PHP.net
manual (http://www.php.net/manual/en/))
Object-oriented programming
Up until version 3, PHP had no object-oriented features. In version 3 basic object functionality was added. The same semantics
were implemented in PHP 4 as well as pass-by-reference and return-by-reference for objects but the implementation still lacked the powerful and useful features of
other object-oriented languages like C++ and Java.
In version 5, which was released in July 2004, PHP's object-oriented functionality has been very much enhanced and is more
robust and complete. Here is a summary of some of the changes in PHP 5 (powered by Zend Engine II
(http://www.zend.com/php5/)):
- New Object Model — PHP's handling of objects has been completely
rewritten, allowing for better performance and more features. In previous
versions of PHP, objects were handled like primitive types (for
instance integers and strings). The
drawback of this method was that semantically the whole object was copied when a variable was assigned, or passed as a parameter
to a method. In the new approach, objects are referenced by handle, and not by value (one can
think of a handle as an object's identifier).
- Private and Protected Members — PHP 5 introduces private and protected member variables, they allow you to define the visibility of class properties.
- Private and Protected Methods — Private and protected methods are also introduced.
- Abstract Classes and Methods — PHP 5 also introduces abstract classes and methods. An abstract method
only declares the method's signature and does not provide an implementation. A class that contains abstract methods needs to be declared abstract.
- Interfaces — A class may implement an arbitrary list of interfaces.
- Object Cloning — If the developer asks to create a copy of an object by using the reserved word clone,
the Zend engine will check if a __clone() method has been defined or not. If not, it will call a default __clone() which will
copy all of the object's properties. If a __clone() method is defined, then it will be responsible to set the necessary
properties in the created object. For convenience, the engine will supply a function that imports all of the properties from the
source object, so that they can start with a by-value replica of the source object, and only override properties that need to be changed.
- Unified Constructors — PHP 5 introduces a standard way of declaring constructor methods by calling them by the name __construct().
- Destructors — PHP 5 introduces a destructor concept similar to
that of other object-oriented languages, such as Java: When the last reference to an object is destroyed, the object's destructor
(a class method named __destruct() that receives no parameters) is called before the object is freed from memory.
- Exceptions — PHP 4 had no exception handling.
PHP 5 introduces an exception model similar to that of other programming languages.
More additions and examples of the additions mentioned above are available in the Classes and
Objects chapter (http://php.net/manual/en/language.oop5.php) of the PHP 5
manual.
It is should be noted that the static method and class variable features in Zend Engine 2 do not work the way some expect.
There is no virtual table feature in the Engine, so the static variables
are bound with a name at compile time instead of with a reference. This can lead to unexpected behavior, if you do not understand
this.
Here is an example of creating an object:
<?php
class Car() {
var $miles;//variable that can be accesed outside the class
function Car($param) { //is called when class "Car" is created
dosomething($param);
}
function startcar() {
startthecar();
}
function stopcar() {
stopthecar();
}
function miles {
return $this->miles;
}
}
$car = new Car($param);
echo $car->miles; //echos the value of the property "miles" of the class "Car"
?>
For more on PHP's OOP abilities, see:
Criticism
Criticism of PHP includes those general criticisms ascribed to other scripting programming languages and dynamically typed languages. In addition, specific criticism of PHP includes:
Syntax
- PHP does not enforce the declaration of variables, and variables that have not been initialized can have operations (such as
concatenation) performed on them; however, an operation on an uninitialized variable does raise an E_NOTICE level error, errors
that are hidden by default. This leads to security holes with register_globals (not on by default), as mentioned below. See also
error_reporting()
(http://php.net/manual/en/function.error-reporting.php).
- Within sections of the built-in function selection there is little or no consistency regarding argument order (examples:
order of subject array and other data for array handling functions, order of needle and haystack in various search
functions).
Built-in functions
- Built-in function names have no standard form, with some employing underscores (strip_tags) while others do not
(stripslashes). Although all new functions do follow a naming standard, old names remain for backward compatibility reasons.
- Some functions have inconsistent output. Statements like This function may return Boolean FALSE, but may also return a
non-Boolean value which evaluates to FALSE, such as 0 or "". can be found in the
documentation (http://php.net/manual/en/function.strpos.php). This is related to
PHP's dynamic typing. A workaround is using strict (===) type checking as opposed to loose (==). See also the manual on type juggling
(http://php.net/manual/en/language.types.type-juggling.php).
- In some areas the selection of built-in functions is notably incomplete (supplying intersection and union functions, but no
full difference function, for example).
- The number of built-in functions is said to be too numerous, with many functions performing the same actions, but with just
slightly different data, results, etc. This is said to make it difficult to program in the language without the frequent
consultation of a reference work.
- There are over 3,000 functions, sharing the same global namespace. Most functions are not enabled by default; available
functions depend on the configuration options.
- Some default settings and features are said to be confusing and the cause of frequent errors.
- There is a "magic quotes" feature that inserts backslashes into user input strings. The feature was introduced to reduce code
written by beginners from being dangerous (such as in SQL injection attacks), but some
criticize it as a frequent cause of improperly displayed text. (Always be sure to check for "magic-quotes":
get_magic_quotes_gpc(); and to unset "magic-quotes-runtime": set_magic_quotes_runtime(0);.) For more
information, see the security section in the Magic Quotes chapter
(http://php.net/manual/en/security.magicquotes.php) of the PHP manual.
Security
- If register_globals
(http://php.net/manual/en/ini.sect.data-handling.php#ini.register-globals) is enabled in PHP's configuration file,
users could cause harm by manipulating poorly written code. As of version 4.2.0
(http://php.net/release_4_2_0.php) register_globals defaults to off. For more information, see the security section
in the Using Register Globals chapter
(http://php.net/manual/en/security.globals.php) of the PHP manual.
- Other languages, such as ASP.NET, include functionality to detect and clean
harmful cross-site scripting or other malicious code
automatically, whereas PHP does not. See also strip_tags()
(http://php.net/manual/en/function.strip-tags.php).
- In the majority of cases, Linux and Unix
webservers with PHP installed (using mod_php) typically run PHP scripts as "nobody", which can make file security in a shared
hosting environment difficult.
Miscellaneous
- Error messages are said to be confusing. This is a common criticism levelled at many programming languages. The error
messages generated by PHP are easier to comprehend than those of Perl. [7]
(http://www.javascriptkit.com/howto/phpcgi.shtml) For further information, see the manual section on PHP parser
tokens (http://php.net/manual/en/tokens.php).
- The many settings in the PHP interpreter's configuration file (php.ini) mean that code that works with one
installation of PHP might not work with another. For example, if code is written to work with register_globals turned on, it
won't work on another system that has register_globals turned off. This makes it necessary to write code that is cross-platform
compatible by assuming that register_globals will be off and therefore calling a global variable with its prefix in front of its
name, such as $_POST['variable'], $_SERVER['variable'] and $_COOKIE['variable']—not, simply, $variable. For more
information, see the manual page on using external variables
(http://php.net/manual/en/language.variables.external.php).
Support
PHP is self-supporting with respect to user support. Direct, one-to-one help is frequently provided free of charge through all
of these and other media. PHP users assist each other through various media such as chat, forums, newsgroups and PHP developer
web sites. In turn, the PHP development team actively participates in such communities, garnering assistance from them in their
own development effort (PHP itself) and providing assistance to them as well. There are many help resources available for the
novice PHP programmer.
These resources include:
Applications built with PHP
The following is a list of notable applications developed using PHP:
External links
PHP home site
- Official PHP website (http://www.php.net/)
- Selected sub-pages of php.net:
- Selected sub-domains of php.net:
Advocacy
Frameworks
Integrated development environments, debuggers and other tools
- Open Directory Project: PHP Tools
(http://www.dmoz.org/Computers/Programming/Languages/PHP/Tools/)
Security
- PHP Security Consortium (http://phpsec.org/) — International group of PHP experts dedicated to promoting secure
programming practices.
- PHP Security
Workbook (http://shiflett.org/php-security.pdf) — A 55-page workbook on
various security topics.
- WACT PHP Application Security Wiki
(http://wact.sourceforge.net/index.php/PhpApplicationSecurity) — The Web Application Component Toolkit's wiki
page on PHP security resources.
Tutorials
- Open Directory Project: PHP Tutorials
(http://www.dmoz.org/Computers/Programming/Languages/PHP/Tutorials/)
Articles and other resources
- Codewalkers.com (http://www.codewalkers.com/) — Community based on PHP and MySQL development.
- Hotscripts.com PHP scripts for webmasters
(http://www.hotscripts.com/PHP/index.html)
- PHP5:
Coming Soon to a Webserver Near You (http://www.sitepoint.com/article/1192/1)
— SitePoint (http://www.sitepoint.com) overview article about upcoming changes in PHP5 by Harry Fuecks, July
28, 2003
- PHPBuilder (http://www.phpbuilder.com/) — Online resource for PHP developers
- PHP Classes
repository (http://www.phpclasses.org/) — Large repository of ready-to-use PHP
components made available by hundreds of PHP developers in the form of source code of classes of objects
- PHP Developer's
Network (http://forums.devnetwork.net/) — Online community of beginners,
intermediates and advanced programmers
- PHP Freaks (http://www.phpfreaks.com/) — Online resource packed with tutorials, full scripts, and
utilities.
- PHP Presents
(http://talks.php.net/) and PHP
Conference Material Site (http://conf.php.net/) — Sites collecting slides of
talks given by well known people from the PHP community. The former one is also known as pres2 (version 2 of the
latter).
- A PHP 4 Coding Standard
(http://www.dagbladet.no/development/phpcodingstandard/)
- PHP Weblog
(http://www.php9.com/) — PHP developer weblog, with articles, code tutorials, news
- Practical PHP
Programming (http://www.hudzilla.org/phpbook/)
- Web Design Reference
(http://www.d.umn.edu/itss/support/Training/Online/webdesign/) — Huge mega-reference (over 3,000 links) of
information and articles dedicated web design and development. The site advocates web standards, accessibility, usability, PHP
and many related topics. It features a listserv and RSS feed for site updates.
Miscellaneous
- Open Directory Project: PHP
(http://www.dmoz.org/Computers/Programming/Languages/PHP/)
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