PHP

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PHP

Usual file extensions	.php
Paradigm	imperative, object-oriented
Appeared in	1995 (1995)
Designed by	Rasmus Lerdorf
Developer	The PHP Group
Latest release	5.2.9/ 2009-02-26; 39 days ago
Latest unstable release	5.3.0RC1, and 6.0-dev[1]
Typing discipline	Dynamic, weak
Major implementations	Roadsend PHP, Phalanger, Quercus, Project Zero
Influenced by	C, Perl, Java, C++, Tcl[citation needed]
Influenced	Php4delphi
OS	Cross-platform
License	PHP License
Website	http://www.php.net/

PHP is a scripting language originally designed for producing dynamic web pages. It has evolved to include a command line interface capability and can be used in standalone graphical applications.^[2]

While PHP was originally created by Rasmus Lerdorf in 1995, the main implementation of PHP is now produced by The PHP Group and serves as the de facto standard for PHP as there is no formal specification.^[3] PHP is free software released under the PHP License, however it is incompatible with the GNU General Public License (GPL), due to restrictions on the usage of the term PHP.^[4]

PHP is a widely-used general-purpose scripting language that is especially suited for web development and can be embedded into HTML. It generally runs on a web server, taking PHP code as its input and creating web pages as output. It can be deployed on most web servers and on almost every operating system and platform free of charge.^[5] PHP is installed on more than 20 million websites and 1 million web servers.^[6]

[edit] History

Rasmus Lerdorf, who wrote the original Common Gateway Interface binaries, and Andi Gutmans and Zeev Suraski, who rewrote the parser that formed PHP 3

PHP originally stood for Personal Home Page.^[3] It began in 1994 as a set of Common Gateway Interface binaries written in the C programming language by the Danish/Greenlandic programmer Rasmus Lerdorf. Lerdorf initially created these Personal Home Page Tools to replace a small set of Perl scripts he had been using to maintain his personal homepage. The tools were used to perform tasks such as displaying his résumé and recording how much traffic his page was receiving.^[3] He combined these binaries with his Form Interpreter to create PHP/FI, which had more functionality. PHP/FI included a larger implementation for the C programming language and could communicate with databases, enabling the building of simple, dynamic web applications. Lerdorf released PHP publicly on June 8, 1995 to accelerate bug location and improve the code.^[7] This release was named PHP version 2 and already had the basic functionality that PHP has today. This included Perl-like variables, form handling, and the ability to embed HTML. The syntax was similar to Perl but was more limited, simpler, and less consistent.^[3]

Zeev Suraski and Andi Gutmans, two Israeli developers at the Technion IIT, rewrote the parser in 1997 and formed the base of PHP 3, changing the language's name to the recursive initialism PHP: Hypertext Preprocessor.^[8] The development team officially released PHP/FI 2 in November 1997 after months of beta testing. Afterwards, public testing of PHP 3 began, 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.^[9] They also founded Zend Technologies in Ramat Gan, Israel.^[3]

On May 22, 2000, PHP 4, powered by the Zend Engine 1.0, was released.^[3] On July 13, 2004, PHP 5 was released, powered by the new Zend Engine II.^[3] PHP 5 included new features such as improved support for object-oriented programming, the PHP Data Objects extension (which defines a lightweight and consistent interface for accessing databases), and numerous performance enhancements.^[10] The most recent update released by The PHP Group is for the older PHP version 4 code branch. As of August, 2008 this branch is up to version 4.4.9. PHP 4 is no longer under development nor will any security updates be released.^[11][12]

In 2008, PHP 5 became the only stable version under development. Late static binding has been missing from PHP and will be added in version 5.3.^[13][14] PHP 6 is under development alongside PHP 5. Major changes include the removal of register_globals,^[15] magic quotes, and safe mode.^[11][16] The reason for the removals was because register_globals had given way to security holes, and magic quotes had an unpredictable nature, and was best avoided. Instead, to escape characters, magic quotes may be substituted with the addslashes() function, or more appropriately an escape mechanism specific to the database vendor itself like mysql_real_escape_string() for MySQL.

PHP does not have complete native support for Unicode or multibyte strings;^[17] Unicode support will be included in PHP 6.^[18] Many high profile open source projects ceased to support PHP 4 in new code as of February 5, 2008, due to the GoPHP5 initiative, provided by a consortium of PHP developers promoting the transition from PHP 4 to PHP 5.^[19][20]

It runs in both 32-bit and 64-bit environments, but on Windows the only official distribution is 32-bit, requiring Windows 32-bit compatibility mode to be enabled while using IIS in a 64-bit Windows environment. There is a third-party distribution^[21] available for 64-bit Windows.

[edit] Release history

[edit] Usage

PHP is a general-purpose scripting language that is especially suited for web development. PHP generally runs on a web server, taking PHP code as its input and creating web pages as output. It can also be used for command-line scripting and client-side GUI applications. PHP can be deployed on most web servers, many operating systems and platforms, and can be used with many relational database management systems. It is available free of charge, and the PHP Group provides the complete source code for users to build, customize and extend for their own use.^[5]

PHP primarily acts as a filter,^[29] taking input from a file or stream containing text and/or PHP instructions and outputs another stream of data; most commonly the output will be HTML. It can automatically detect the language of the user.^[30][31] From PHP 4, the PHP parser compiles input to produce bytecode for processing by the Zend Engine, giving improved performance over its interpreter predecessor.^[32]

Originally designed to create dynamic web pages, PHP's principal focus is server-side scripting,^[33] and it is similar to other server-side scripting languages that provide dynamic content from a web server to a client, such as Microsoft's Active Server Pages, Sun Microsystems' JavaServer Pages,^[34] and mod_perl. PHP has also attracted the development of many frameworks that provide building blocks and a design structure to promote rapid application development (RAD). Some of these include CakePHP, Symfony, CodeIgniter, and Zend Framework, offering features similar to other web application frameworks.

The LAMP architecture has become popular in the web industry as a way of deploying web applications. PHP is commonly used as the P in this bundle alongside Linux, Apache and MySQL, although the P may also refer to Python or Perl.

As of April 2007, over 20 million Internet domains were hosted on servers with PHP installed, and PHP was recorded as the most popular Apache module.^[35] Significant websites are written in PHP including the user-facing portion of Facebook,^[36] Wikipedia (MediaWiki),^[37] Yahoo!,^[38] MyYearbook,^[39], Digg, WordPress and Tagged.^[40]

In addition to server-side scripting, PHP can be used to create stand-alone, compiled applications and libraries, it can be used for shell scripting, and the PHP binaries can be called from the command line.

[edit] Speed optimization

As with many scripting languages, PHP scripts are normally kept as human-readable source code, even on production web servers.^[41] In this case, PHP scripts will be compiled at runtime by the PHP engine, which increases their execution time. PHP scripts are able to be compiled before runtime using PHP compilers as with other programming languages such as C (the language PHP and its extensions are written in).

Code optimizers aim to reduce the computational complexity of the compiled code by reducing its size and making other changes that can reduce the execution time with the overall goal of improving performance. The nature of the PHP compiler is such that there are often opportunities for code optimization,^[42] and an example of a code optimizer is the Zend Optimizer PHP extension.^[43]

Another approach for reducing overhead for high load PHP servers is using PHP accelerators. These can offer significant performance gains by caching the compiled form of a PHP script in shared memory to avoid the overhead of parsing and compiling the code every time the script runs.

[edit] Security

The National Vulnerability Database stores all vulnerabities found in computer software. The overall proportion of PHP-related vulnerabilities on the database amounted to: 12% in 2003, 20% in 2004, 28% in 2005, 43% in 2006, 36% in 2007, and 35% in 2008.^[44] Most of these PHP-related vulnerabilities can be exploited remotely: they allow crackers to steal or destroy data from data sources linked to the webserver (such as an SQL database), send spam or contribute to DOS attacks using malware, which itself can be installed on the vulnerable servers.

These vulnerabilities are caused mostly by not following best practice programming rules: technical security flaws of the language itself or of its core libraries are not frequent. ^[45][46] Recognizing that programmers cannot be trusted, some languages include taint checking to detect automatically the lack of input validation which induces many issues. Such a feature is being developed for PHP.^[47] Although it may be included in mainstream PHP in a future release, its inclusion has been rejected several times in the past.^[48][49]

Hosting PHP applications on a server requires a careful and constant attention to deal with these security risks.^[50] There are advanced protection patches such as Suhosin and Hardening-Patch, especially designed for web hosting environments.^[51] Installing PHP as a CGI binary rather than as an Apache module is the preferred method for added security.^[52]

With respect to securing the code itself, PHP code can be obfuscated to make it difficult to read while remaining functional.

[edit] Syntax

Syntax-highlighted PHP code embedded within HTML

PHP only parses code within its delimiters. Anything outside its delimiters is sent directly to the output and is not parsed by PHP. The most common delimiters are <?php and ?>, which are open and close delimiters respectively. <script language="php"> and </script> delimiters are also available. Short tags can be used to start PHP code, <? or <?= (which is used to echo back a string or variable) and the tag to end PHP code, ?>. These tags are commonly used, but like ASP-style tags (<% or <%= and %>), they are less portable as they can be disabled in the PHP configuration. For this reason, the use of short tags and ASP-style tags is discouraged.^[53] The purpose of these delimiters is to separate PHP code from non-PHP code, including HTML.^[54]

Variables are prefixed with a dollar symbol and a type does not need to be specified in advance. Unlike function and class names, variable names are case sensitive. Both double-quoted ("") and heredoc strings allow the ability to embed a variable's value into the string.^[55] PHP treats newlines as whitespace in the manner of a free-form language (except when inside string quotes), and statements are terminated by a semicolon.^[56] PHP has three types of comment syntax: /* */ serves as block comments, and // as well as # are used for inline comments.^[57] The echo statement is one of several facilities PHP provides to output text (e.g. to a web browser).

In terms of keywords and language syntax, PHP is similar to most high level languages that follow the C style syntax. If conditions, for and while loops, and function returns are similar in syntax to languages such as C, C++, Java and Perl.

[edit] Data types

PHP stores whole numbers in a platform-dependent range. This range is typically that of 32-bit signed integers. Unsigned integers are converted to signed values in certain situations; this behavior is different from other programming languages.^[58] Integer variables can be assigned using decimal (positive and negative), octal, and hexadecimal notations. Floating point numbers are also stored in a platform-specific range. They can be specified using floating point notation, or two forms of scientific notation.^[59] PHP has a native Boolean type that is similar to the native Boolean types in Java and C++. Using the Boolean type conversion rules, non-zero values are interpreted as true and zero as false, as in Perl and C++.^[59] The null data type represents a variable that has no value. The only value in the null data type is NULL.^[59] Variables of the "resource" type represent references to resources from external sources. These are typically created by functions from a particular extension, and can only be processed by functions from the same extension; examples include file, image, and database resources.^[59] Arrays can contain elements of any type that PHP can handle, including resources, objects, and even other arrays. Order is preserved in lists of values and in hashes with both keys and values, and the two can be intermingled.^[59] PHP also supports strings, which can be used with single quotes, double quotes, or heredoc syntax.^[60]

The Standard PHP Library (SPL) attempts to solve standard problems and implements efficient data access interfaces and classes. ^[61]

[edit] Functions

PHP has hundreds of base functions and thousands more via extensions. These functions are well documented on the PHP site, but unfortunately, the built-in library has a wide variety of naming conventions and inconsistencies.^[62] PHP currently has no functions for thread programming.

[edit] 5.2 and earlier

Functions are not first-class functions and can only be referenced by their name--directly or dynamically by a variable containing the name of the function. ^[63] User-defined functions can be created at any time without being prototyped.^[63] Functions can be defined inside code blocks, permitting a run-time decision as to whether or not a function should be defined. Function calls must use parentheses, with the exception of zero argument class constructor functions called with the PHP new operator, where parentheses are optional. PHP supports quasi-anonymous functions through the create_function() function, although they are not true anonymous functions because anonymous functions are nameless, but functions can only be referenced by name, or indirectly through a variable $function_name();, in PHP.^[63]

[edit] 5.3 and newer

PHP gained support for first-class functions and closures. True anonymous functions are supported using the following syntax:

function getAdder($x)
{
    return function ($y) use ($x) {
        return $x + $y;
    };
}
 
$adder = getAdder(8);
echo $adder(2); // prints "10"

Here, getAdder() function creates a closure using parameter $x (keyword "use" forces getting variable from context), which takes additional argument $y and returns it to the caller. Such a function can be stored, given as the parameter to another functions, etc. For more details see Lambda functions and closures RFC.

[edit] Objects

Basic object-oriented programming functionality was added in PHP 3.^[3] Object handling was completely rewritten for PHP 5, expanding the feature set and enhancing performance.^[64] In previous versions of PHP, objects were handled like primitive types.^[64] The drawback of this method was that 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. PHP 5 introduced private and protected member variables and methods, along with abstract classes and final classes as well as abstract methods and final methods. It also introduced a standard way of declaring constructors and destructors, similar to that of other object-oriented languages such as C++, and a standard exception handling model. Furthermore, PHP 5 added interfaces and allowed for multiple interfaces to be implemented. There are special interfaces that allow objects to interact with the runtime system. Objects implementing ArrayAccess can be used with array syntax and objects implementing Iterator or IteratorAggregate can be used with the foreach language construct. There is no virtual table feature in the engine, so static variables are bound with a name instead of a reference at compile time.^[65]

If the developer creates a copy of an object 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 the object's properties. If a __clone() method is defined, then it will be responsible for setting the necessary properties in the created object. For convenience, the engine will supply a function that imports the properties of the source object, so that the programmer can start with a by-value replica of the source object and only override properties that need to be changed.^[66]

[edit] Resources

PHP includes 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 such as embedded PostgreSQL, MySQL and SQLite, LDAP servers, and others. Many functions familiar to C programmers such as those in the stdio family are available in the standard PHP build.^[67] PHP has traditionally used features such as "magic_quotes_gpc" and "magic_quotes_runtime" which attempt to escape apostrophes (') and quotes (") in strings in the assumption that they will be used in databases, to prevent SQL injection attacks. This leads to confusion over which data is escaped and which is not, and to problems when data is not in fact used as input to a database and when the escaping used is not completely correct.^[68] To make code portable between servers which do and do not use magic quotes, developers can preface their code with a script to reverse the effect of magic quotes when it is applied.^[69]

PHP allows developers to write extensions in C to add functionality to the PHP language. These can then be compiled into PHP or loaded dynamically at runtime. Extensions have been written to add support for the Windows API, process management on Unix-like operating systems, multibyte strings (Unicode), cURL, and several popular compression formats. Some more unusual features include integration with Internet Relay Chat, dynamic generation of images and Adobe Flash content, and even speech synthesis. The PHP Extension Community Library (PECL) project is a repository for extensions to the PHP language.^[70]

Zend provides a certification exam for programmers to become certified PHP developers.

[edit] See also

Free software portal

• Active Server Pages
• Comparison of programming languages
• LAMP (software bundle)
• List of PHP editors
• List of web application frameworks
• Perl
• PHP accelerator
• Python (programming language)
• XAMPP (WebServer for PHP)
• Zend Framework
• Zend Certified Engineer

[edit] Notes

[edit] External links

Wikibooks has a book on the topic of Programming:PHP

Wikiversity has learning materials about PHP

• PHP: Hypertext Preprocessor
• PHP at the Open Directory Project
• PHP Reference Manual

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