PHP (PHP:Hypertext Preprocessor) is a reflective programming language originally designed for producing dynamic web pages.(1) PHP is used mainly in server-side scripting, but can be used from a command line interface or in standalone graphical applications. Textual User Interfaces can also be created using ncurses.
The main implementation is produced by "The PHP Group" and released under the PHP License. It is considered to be free software by the Free Software Foundation(2). This implementation serves to define a de facto standard for PHP, as there is no formal specification.
HistoryPHP was written as a set of CGI binaries in the C programming language by the Danish/Greenlandic programmer Rasmus Lerdorf in 1994, to replace a small set of Perl scripts he had been using to maintain his personal homepage.(3) Lerdorf initially created PHP to display his résumé and to collect certain data, such as how much traffic his page was receiving. "Personal Home Page Tools" was publicly released on June 8 1995 after Lerdorf combined it with his own Form Interpreter to create PHP/FI (this release is considered PHP version 2).(4)(5)
Zeev Suraski and Andi Gutmans, two Israeli developers at the Technion - Israel Institute of Technology, rewrote the parser in 1997 and formed the base of PHP 3, changing the language's name to the recursive initialism "PHP: Hypertext Preprocessor". The development team officially released PHP/FI 2 in November 1997 after months of beta testing. 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.(6) They also founded Zend Technologies in Ramat Gan, Israel, which actively manages the development of PHP.
In May 2000, PHP 4, powered by the Zend Engine 1.0, was released. The latest version as of March 2007 is 4.4.6. PHP 4 is currently still supported by security updates for those applications that require it.
On July 13 2004, PHP 5 was released powered by the new Zend Engine II. PHP 5 included new features such as:(7)
- Robust support for Object-Oriented Programming
- The PHP Data Objects extension, which defines a lightweight and consistent interface for accessing databases
- Performance enhancements taking advantage of the new engine
- Better support for MySQL through a completely rewritten extension
- Embedded support for SQLite
- Integrated SOAP support
- Data iterators
- Error handling through Exceptions
The latest stable version, PHP 5.2.1, was released in February 2007.
UsagePHP generally runs on a web server, taking PHP code as its input and creating Web pages as output, however it can also be used for command-line scripting and client-side GUI applications. PHP can be deployed on most web servers and on almost every OS platform free of charge. The PHP Group also provides the complete source code for users to build, customize and extend for their own use.
Server-Side ScriptingOriginally designed to create dynamic web pages, PHP's principal focus is server-side scripting. While running the PHP parser with a web server and web browser, the PHP model can be compared to other server-side scripting languages such as Microsoft's ASP.NET system, Sun Microsystems' JavaServer Pages, mod_perl and the Ruby on Rails framework, as they all provide dynamic content to the client from a web server. To more directly compete with the "framework" approach taken by these systems, Zend is working on the Zend Framework - an emerging (as of June 2006) set of PHP building blocks and best practices; other PHP frameworks along the same lines include CakePHP, PRADO and Symfony.
The LAMP architecture has become popular in the Web industry as a way of deploying inexpensive, reliable, scalable, secure web applications. PHP is commonly used as the P in this bundle alongside Linux, Apache and MySQL. PHP can be used with a large number of relational database management systems, runs on all of the most popular web servers and is available for many different operating systems. This flexibility means that PHP has a wide installation base across the Internet; over 19 million Internet domains are currently hosted on servers with PHP installed.(8) Examples of popular server-side PHP applications include phpBB, WordPress, GetWiki, and many others.
Command-Line ScriptingPHP also provides a command line interface SAPI for developing shell and desktop applications, daemons, log parsing, or other system administration tasks. It is increasingly used on the command line for tasks that have traditionally been the domain of Perl, Python, awk, or shell scripting.
Client-Side GUI ApplicationsPHP provides bindings to GUI libraries such as GTK+ and text mode libraries like ncurses in order to facilitate development of a broader range of cross-platform GUI applications.
SyntaxPHP primarily acts as a filter. The PHP program takes input from a file or stream containing text and special PHP instructions and outputs another stream of data for display. From PHP 4, the PHP parser compiles input to produce bytecode for processing by the Zend Engine, giving improved performance over its interpreter predecessor. The Zend Engine II is at the heart of PHP 5.
The usual Hello World code example for PHP is:
echo 'Hello, World!';
PHP only parses code within its delimiters, such as . Anything outside its delimiters is sent directly to the output and not parsed by PHP. The example above is equivalent to the following text (and indeed is converted into this form):
The primary use of this is to allow PHP statements to be embedded within HTML documents, for example:
// PHP statements here
Regular HTML here
// More PHP Statements
Regular HTML here
// More PHP Statements
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 the variable's value into the string.
PHP treats new lines as whitespace, in the manner of a free-form language (except when inside string quotes). Statements are terminated by a semicolon, except in a few special cases.
PHP has three types of comment syntax: /* */ which serves as block comments, and // as well as # which is used for inline comments.
Data TypesPHP stores whole numbers in a platform-dependent range. This range is typically that of 32-bit signed integers. Integer variables can be assigned using decimal (positive and negative), octal and hexadecimal notations. Real numbers are also stored in a platform-specific range. They can be specified using floating point notation, or two forms of scientific notation.
PHP has a native Boolean type, named "boolean", similar to the native Boolean types in Java and C++. Using the Boolean type conversion rules, non-zero values can be interpreted as true and zero as false, as in Perl.
The null data type represents a variable that has no value. The only value in the null data type is NULL.
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.
Arrays support both numeric and string indices, and are heterogeneous. 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.
ObjectsBasic Object-oriented programming functionality was added in PHP 3. However, handling of objects was completely rewritten for PHP 5, allowing for better performance and more features. In previous versions of PHP, objects were handled like primitive types. 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. PHP 5 introduced private and protected member variables and methods, along with abstract classes and abstract methods. It also introduced a standard way of declaring constructors and destructors similar to that of other object-oriented languages, such as C++, and an exception handling model similar to that of other programming languages.
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.
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 (wiktionary:replica|replica) of the source object, and only override properties that need to be changed.
LibrariesPHP 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 such as embedded 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.
ExtensionPHP 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, and dynamic generation of images and Adobe Flash content. The PHP Extension Community Library (PECL) project is a repository for extensions to the PHP language.
Sourcecode Encoders, Optimizers and AcceleratorsAs with many scripting languages, PHP scripts are normally kept as human-readable source code, even on production webservers. While this allows flexibility, it can raise issues with security and performance.
Encoders offer some source code security and enable proprietary software by hindering source code reverse engineering. Encoders fall broadly into two types; those that hide source code and those that compile code into bytecode. The downside of this latter approach is that a special extension has to be installed on the server in order to run encoded scripts, however the approach of encoding compiled code and use of an extension offers typically the best performance, security and opportunity for additional features that may be useful for developers. Compiled code solutions may exploit the potential for increased security through the use of their own execution engine, although some simpler solutions rely on the regular PHP engine to execute the compiled code. The most commonly used packages for source code protection are from Zend Technologies and ionCube Ltd.
Code optimizers improve the quality of the compiled code by reducing its size and making changes that can reduce the execution time and improve performance. The nature of the PHP compiler is such that there are often many opportunities for code optimization.
Accelerators offer 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 is run. They may also perform code optimization to provide increased execution performance. Both commercial (e.g. Zend Platform) and open source accelerators (e.g. xcache, eAccelerator, APC) are available.
Debuggers and ProfilersDebuggers and profilers allow developers to analyze running PHP code for potential and noted software bugs and bottlenecks. Examples of such software for PHP include APD and Xdebug.
Templating EnginesTemplating engines provide macros that allow PHP applications to uniformly identify common variables. One popular templating engine is Smarty.
PEARThe PHP Extension and Application Repository (PEAR) project aims to provide reusable libraries and components for PHP development. PEAR projects are usually written in PHP code using the Object-oriented programming paradigm.
SupportPHP has a formal development manual that is maintained by the free software community. In addition, answers to many questions can often be found by doing a simple internet search. 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.
CriticismCriticisms of PHP include those general criticisms ascribed to other scripting languages and dynamically typed languages. This list includes criticisms that have been rectified in recent versions.
- PHP originally inserted data received over the network directly into the language namespace ("register_globals"), leading to confusion between trusted and untrusted data, and unnecessary potential for security holes in PHP applications. This behaviour was turned off by default from version 4.2.0 released in April 2002.(9) However, this feature is still being used by some legacy applications(10).
- 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. (11)
- PHP does not have native support for Unicode or multibyte strings. (12)
- PHP does not enforce the declaration of variables prior to their use, and variables which have not been initialized can have operations (such as concatenation) performed on them; an operation on an uninitialized variable raises an E_NOTICE level error, but this is hidden by default.
- PHP has no namespace support.
- As of March 28, 2007, there are 5,312 functions in PHP. (13) This primarily due to the lack of namespaces support.
- PHP's dynamic type conversion could potentially cause problems. Variable types in PHP, although they exist, are transparent to the programmer. Some may consider this a feature, as a variable can change from int to double and back again without extra lines of code. However, variable type errors are not detected at compile-time, and the dynamic-typing behavior lacks full predictability.
- The standard function library lacks internal consistency. Many functions perform relatively similar actions and have different name standards and argument orders. For example:
- Argument consistency: strpos($haystack, $needle) vs. in_array($needle, $haystack)
- Naming convention: both of these work case-insensitively strcasecmp() vs. stristr() but the former indicates this with "case" while the later does with "i"
- Function name consistency: strpos() vs. str_replace()
- Lack of late static binding (14)
- Some portability issues with 32-bit & 64-bit integers, and sometimes unsigned integers get converted to signed values. (15)
Future DevelopmentPHP 6, in development as of October 2006, aims to address some of PHP 5's shortcomings.(16)
- Native Unicode support will be added;
- The magic_quotes option will be removed;
- The HTTP__VARS option will be removed;
- The register_globals option will be removed;
- The safe_mode option will be removed.
In addition, there has been discussion of adding namespace support.(17)
- Associative arrays in PHP
- Comparison of programming languages
- List of PHP editors
- List of web application frameworks
- Paamayim Nekudotayim - Scope Resolution Operator (::)
- PHP accelerator
- Roadsend PHP - Roadsend PHP, an open source implementation of PHP.
- Standard PHP Library
Notes and References
- 2006-09-13, Is PHP The Cure For The 'Broken' Web?, internetnews.com, Kerner, Sean Michael ,weblink
- 2006-10-31, Microsoft Opens PHP Door, internetnews.com, Kerner, Sean Michael,weblink
- Jason E. Sweat, Guide to PHP Design Patterns, PHP&124;architect, 2005, ISBN 0-9735898-2-5
- Ilia Alshanetsky, Guide to PHP Security, PHP&124;architect, 2005, ISBN 0-9738621-0-6
- Chris Shiflett, Essential PHP Security, O'Reilly Media, 2005, ISBN 0-596-00656-X
- Larry Ullman, PHP and MySQL for Dynamic Web Sites, Peachpit Press, 1st Edition, 2003, ISBN 0-321-18648-6
- weblink | work = PHP Manual| title = Introduction | accessdate = 2006-11-15
- GPL-Incompatible, Free Software Licenses | url =weblink | work = Various Licenses and Comments about Them | publisher = Free Software Foundation
- Rasmus Lerdorf | title = Re: There ARE other scriping languages besides PHP | publisher = Slashdot.org | date = 2006-09-15 | url =weblink | accessdate = 2006-09-15
- Announce: Personal Home Page Tools (PHP Tools) | author = Lerdorf, Rasmus | date = 1995-06-08 | newsgroup = comp.infosystems.www.authoring.cgi | url =weblink | accessdate = 2006-09-17
- Zend Engine version 2.0: Feature Overview and Design | publisher = Zend Technologies Ltd. | url =weblink | accessdate = 2006-09-17
- PHP Manual: Chapter 29. Using Register Globals
- Rasmus Lerdorf's thoughts about PHP6
- Overview of PHP function which are currently Unicode compatible (in CVS)
- weblink retrieved March 28, 2007
- weblink retrieved March 28, 2007
- weblink retrieved March 28, 2007
- PHP Documentation
- PHP - An active listing of PHP links
- Patterns for PHP - A pattern collection site
- Practical PHP Programming wiki - An online PHP book
- Do You PHP? by Rasmus Lerdorf
- PHP at W3Schools
- Quercus - An open-source Java implementation of PHP in the Resin application server
Some content adapted from the Pseudopedia article "PHP" under the GNU Free Documentation License.
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