PHP

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For other uses, see PHP (disambiguation).

PHP is an open-source, reflective programming language used mainly for developing server-side applications and dynamic web content, and more recently, other software. Originally, PHP stood for "Personal Home Page". Today, the official meaning is the recursive acronym "PHP: Hypertext Preprocessor". Some fans of this language also use the label "Pretty Hypertext Preprocessor".

Examples of PHP applications include phpBB as well as MediaWiki. 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, the Zope/Python system, and to the Mod perl/Perl system.

PHP

PHP logo
Maintainer: The PHP Group
Latest release: 4.4.1
5.0.5
5.1.0RC1 / September 9, 2005
OS: Cross-platform
Genre: Scripting programming language
License: PHP License 3.0
Website: www.php.net

PHP allows interaction with a large number of relational database management systems, such as MySQL, Oracle, IBM DB2, Microsoft SQL Server, PostgreSQL and SQLite. 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 contains extensive documentation. 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.) Alternatively, the Windows, IIS, MySQL, PHP (WIMP) and Windows, Apache, MySQL, PHP (WAMP) architectures exist as alternatives for those wishing to use Windows as their server operating system.

PHP is the result of the efforts of many contributors. It is licensed under a BSD-style license, the PHP license. Since version 4, it has been powered by the Zend engine.

Contents

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History

PHP was originally designed as a small set of Perl scripts, followed by a rewritten set of CGI binaries written in C by the Danish-Canadian programmer Rasmus Lerdorf in 1994 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 8th June 1995 after Lerdorf 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, changing the language's name to its current recursive form. The development team officially released PHP/FI 2 in November 1997 after months of beta testing. 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 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). PHP 5 includes new features such as PHP Data Objects (PDO) and more performance enhancements taking advantage of the new Zend Engine II.

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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 are not as strict with variables—they do not have to be declared and they can hold any type of object. Arrays are heterogenous, meaning a single array can contain objects of more than one type.

According to Netcraft's April 2002 survey, PHP is now the most deployed server-side scripting language, running on around 9 million 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]

Due to PHP's popularity in the web space, a new breed of programmers emerged who are familiar only with PHP. This encouraged the development of a command line interface for PHP, as well as GUI libraries such as GTK+ and text mode libraries like Ncurses and Newt. This was a major step for PHP, because it helped move it from being a language used only for CGI to a general-purpose programming language. On the desktop it has been favored by some new programmers as a rapid prototyping environment.

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Code example

Here is a Hello World code example (See Basic syntax in the PHP manual):

<?php
 echo 'Hello World!';
?>

Here is an example that prints out the lyrics for the song 99 Bottles of Beer:

<?php
   /*
   *  This block 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 '' (an empty string) for one 
   // and 's' for all other numbers
}

// We define a variable called $lb to contain an 
// XHTML line break and a newline :
$lb = "<br />\n"; 

// Be sure to count DOWN
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!';
?>

Here's an alternate way of doing the previous example: (with simpler PHP code)

<html>
<head>
<title>99 Bottles of Beer on the Wall</title>
</head>

<body>
<h1>99 Bottles of Beer on the Wall</h1>
<p>
<?php
$beer = 99;

while($beer > 2)
{

print $beer.' bottles of beer on the wall, '.$beer.' bottles of beer.<br />'."\r\n";
print 'Take one down, pass it around,<br />'."\r\n";
$beer--;
print $beer.' bottles of beer on the wall.<br />'."\r\n";

}

/*
Note that characters such as return (\r) and newline (\n) must be used
inside "double quoted" strings. If placed in a 'single quoted' string, the 
character will be rendered in its literal form. 
*/

?>
2 bottles of beer on the wall, 2 bottles of beer.<br />
Take one down, pass it around,<br />
1 bottle of beer on the wall. <br/ >
1 bottle of beer on the wall, 1 bottle of beer.<br />
Take one down, pass it around,<br />
No more bottles of beer on the wall.<br /><br />
Go to the store,<br />
buy some more,<br />
99 bottles of beer on the wall!
</p>
</body>
</html>

Notes:

  • PHP treats new lines as whitespace, in the manner of a free-form language (except when inside string quotes). A line of code is terminated only by a semicolon (;) except in a few special cases. [2]
  • A period (.) concatenates strings together. [3]
  • Variables are case sensitive and always have names that start with a dollar sign ($), and are evaluated inside double quotation marks ("), but not inside single quotation marks ('). Functions, such as plural(), and other expressions are not evaluated inside double quotes but can be added to strings using periods for concatenation.

Example:

$var = 'string' . function() . 'rest of string';

[4]

  • Although PHP allows both # and // for "same line" comments, it is generally preferred to use the C++-style // and not the Bourne Shell style #. [5]
  • Also seen above are the C style block comments starting with /* and ending with */. All text within these tags are ignored
  • For output, this program uses echo. print and printf may also be used for this purpose.
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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)

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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):

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 abstract 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 an abstract class.
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 C++: 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 of the PHP 5 manual.

It 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 {
   public $miles; //variable that can be accessed outside the class
   private $mpg; //variable that can only be accessed within the class
   protected $mph; //variable that can only be accessed from within the class, and                  
                  //from any inherited child classes

   function __construct($param) { //is called when object "Car" is created
      dosomething($param);
   }

   public function startcar() { //methods can be public, private, or protected
      startthecar();
   }

   public function stopcar() {
      stopthecar();
   }

   public 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:

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Criticisms

Criticisms of PHP include those general criticisms ascribed to other scripting programming languages and dynamically typed languages. In addition, specific criticisms of PHP include:

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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().
  • 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).
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Built-in functions

  • Built-in function names have no standard form, with some employing underscores (e.g. strip_tags, html_entity_decode) while others do not (e.g. stripslashes, htmlentities). Furthermore, some functions are verb_noun() while others are noun_verb() and some are prefixed_byModuleName while others use a module_suffix_scheme. 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. 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.
  • 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, but become available when PHP is linked against the required libraries. To mitigate this, function names are usually prefixed with their library name.
  • 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 or encouraging beginners to write PHP which is vulnerable to SQL-injection when used on a system with it turned off. (Always be sure to check for "magic-quotes": get_magic_quotes_gpc(); and to unset "magic-quotes-runtime": set_magic_quotes_runtime(0);.) Magic Quotes are turned off by default in PHP 5. For more information, see the security section in the Magic Quotes chapter of the PHP manual.
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Security

  • If register_globals is enabled in PHP's configuration file, users could cause harm by manipulating poorly written code. As of version 4.2.0 register_globals defaults to off. For more information, see the security section in the Using Register Globals chapter 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().
  • 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.
  • PHP has no variable tainting mechanism (which is very useful for a language designed to accept and process untrusted input.)
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Miscellaneous

  • Error messages are said to be confusing, although this is a common criticism levelled at many programming languages. For further information, see the manual section on PHP parser tokens.
  • 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.
  • Some PHP extensions use libraries that are not threadsafe, so rendering with Apache 2's Multithreaded MPM (multi-processing module) may cause crashes.
  • There is no native support for Unicode or multibyte strings (mbstring is provided as an extension). This major improvement is planned for the next major revision (5.5 or 6.0).
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Support

PHP has a formal development manual that is maintained by the open source community. In addition, answers to most 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.

These resources include:

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See also

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External links

Wikibooks
Wikibooks has more about this subject:
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PHP.net (official home of PHP)

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Advocacy

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Books

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Resources

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Security


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Miscellaneous


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