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<h2>Syntax</h2>
<p>PHP only parses code within its <a href="http://en.wikipedia.org/wiki/Delimiter" title="Delimiter">delimiters</a>. Anything outside its delimiters is sent directly to the output and is not parsed by PHP. The most common delimiters are <span s><?php</span> and <span>?></span>, which are open and close delimiters respectively. <span><script language="php"></span> and <span></script></span> delimiters are also available. Short tags (<span><?</span> or <span><?=</span> and <span>?></span>) are also commonly used, but like ASP-style tags (<span><%</span> or <span><%=</span> and <span>%></span>), 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. The purpose of these delimiters is to separate PHP code from non-PHP code, including HTML. Everything outside the delimiters is ignored by the parser and is passed through as output.</p>
<p>Variables are prefixed with a <a href="http://en.wikipedia.org/wiki/Dollar_sign" title="Dollar sign">dollar symbol</a> and a <a href="http://en.wikipedia.org/wiki/Primitive_type" title="Primitive type">type</a> does not need to be specified in advance. Unlike function and class names, variable names are case sensitive. Both double-quoted (<span>""</span>) and <a href="http://en.wikipedia.org/wiki/Heredoc" class="mw-redirect" title="Heredoc">heredoc</a> strings allow the ability to embed the variable's value into the string. PHP treats <a href="http://en.wikipedia.org/wiki/Newline" title="Newline">newlines</a> as <a href="http://en.wikipedia.org/wiki/Whitespace_%28computer_science%29" title="Whitespace (computer science)">whitespace</a> in the manner of a <a href="http://en.wikipedia.org/wiki/Free-form_language" title="Free-form language">free-form language</a> (except when inside string quotes), and statements are terminated by a semicolon. PHP has three types of <a href="http://en.wikipedia.org/wiki/Comparison_of_programming_languages_%28syntax%29#Comments" title="Comparison of programming languages (syntax)">comment syntax</a>: <span>/* */</span> serves as block comments, and <span>//</span> as well as <span>#</span> are used for inline comments. To output text to the browser, either the <tt>print</tt> function or the <tt>echo</tt> function is used. Both functions are nearly identical; the major difference is that <tt>print</tt> is slower than <tt>echo</tt> because the former will return a status indicating if it was successful or not, whereas the latter does not return a status and only returns the text for output.</p>
<p><a name="Data_types" id="Data_types"></a></p>
<h3><span class="mw-headline">Data types</span></h3>
<p>PHP stores whole numbers in a platform-dependent range. This range is typically that of 32-bit <a href="http://en.wikipedia.org/wiki/Signed_number_representations" title="Signed number representations">signed</a> <a href="http://en.wikipedia.org/wiki/Integer_%28computer_science%29" title="Integer (computer science)">integers</a>. Unsigned integers are converted to signed values in certain situations; this behavior is different from other programming languages. Integer variables can be assigned using decimal (positive and negative), <a href="http://en.wikipedia.org/wiki/Octal" title="Octal">octal</a>, and <a href="http://en.wikipedia.org/wiki/Hexadecimal" title="Hexadecimal">hexadecimal</a> notations. <a href="http://en.wikipedia.org/wiki/Real_numbers" class="mw-redirect" title="Real numbers">Real numbers</a> are also stored in a platform-specific range. They can be specified using <a href="http://en.wikipedia.org/wiki/Floating_point" title="Floating point">floating point</a> notation, or two forms of <a href="http://en.wikipedia.org/wiki/Scientific_notation" title="Scientific notation">scientific notation</a>. PHP has a native <a href="http://en.wikipedia.org/wiki/Boolean_datatype" title="Boolean datatype">Boolean</a> type that is similar to the native Boolean types in <a href="http://en.wikipedia.org/wiki/Java_%28programming_language%29" title="Java (programming language)">Java</a> and <a href="http://en.wikipedia.org/wiki/C%2B%2B" title="C++">C++</a>. Using the Boolean type conversion rules, non-zero values are interpreted as true and zero as false, as in Perl and C++. The null data type represents a variable that has no value. The only value in the null data type is <i>NULL</i>. 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 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 <a href="http://en.wikipedia.org/wiki/Hash_table" title="Hash table">hashes</a> with both keys and values, and the two can be intermingled. PHP also supports <a href="http://en.wikipedia.org/wiki/String_%28computing%29" class="mw-redirect" title="String (computing)">strings</a>, which can be used with single quotes, double quotes, or <a href="http://en.wikipedia.org/wiki/Heredoc" class="mw-redirect" title="Heredoc">heredoc syntax</a>.</p>
<p><a name="Functions" id="Functions"></a></p>
<h3><span class="mw-headline">Functions</span></h3>
<p>PHP has hundreds of base functions and thousands more from extensions. Functions are not <a href="http://en.wikipedia.org/wiki/First-class_function" title="First-class function">first-class functions</a> and can only be referenced by their name. User-defined functions can be created at any time without being prototyped. Functions can be defined inside code blocks, permitting a <a href="http://en.wikipedia.org/wiki/Dynamic_dispatch" title="Dynamic dispatch">run-time decision</a> as to whether or not a function should be defined. Function calls must use parentheses, with the exception of zero argument class <a href="http://en.wikipedia.org/wiki/Constructor_%28computer_science%29" title="Constructor (computer science)">constructor</a> functions called with the PHP <span>new</span> operator, where parentheses are optional. PHP supports quasi-<a href="http://en.wikipedia.org/wiki/Anonymous_function" title="Anonymous function">anonymous functions</a> through the <span>create_function()</span> 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 <span>$function_name();</span>, in PHP.</p>
<p><a name="Objects" id="Objects"></a></p>
<h3><span class="mw-headline">Objects</span></h3>
<p>Basic <a href="http://en.wikipedia.org/wiki/Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a> functionality was added in PHP 3. Object handling was completely rewritten for PHP 5, expanding the feature set and enhancing performance. In previous versions of PHP, objects were handled like <a href="http://en.wikipedia.org/wiki/Primitive_type" title="Primitive type">primitive types</a>. 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 <a href="http://en.wikipedia.org/wiki/Smart_pointer#Handles" title="Smart pointer">handle</a>, and not by value. PHP 5 introduced private and protected <a href="http://en.wikipedia.org/wiki/Member_variable" class="mw-redirect" title="Member variable">member variables</a> and methods, along with <a href="http://en.wikipedia.org/wiki/Abstract_type" title="Abstract type">abstract classes</a> and <a href="http://en.wikipedia.org/wiki/Final_type" class="mw-redirect" title="Final type">final classes</a> as well as <a href="http://en.wikipedia.org/wiki/Abstract_method" class="mw-redirect" title="Abstract method">abstract methods</a> and <a href="http://en.wikipedia.org/wiki/Final_method" class="mw-redirect" title="Final method">final methods</a>. It also introduced a standard way of declaring <a href="http://en.wikipedia.org/wiki/Constructor_%28computer_science%29" title="Constructor (computer science)">constructors</a> and <a href="http://en.wikipedia.org/wiki/Destructor_%28computer_science%29" title="Destructor (computer science)">destructors</a>, similar to that of other object-oriented languages such as <a href="http://en.wikipedia.org/wiki/C%2B%2B" title="C++">C++</a>, and a standard <a href="http://en.wikipedia.org/wiki/Exception_handling" title="Exception handling">exception handling</a> model. Furthermore, PHP 5 added <a href="http://en.wikipedia.org/wiki/Interfaces" class="mw-redirect" title="Interfaces">interfaces</a> and allowed for multiple interfaces to be implemented. There are special interfaces that allow objects to interact with the runtime system. <a href="http://en.wikipedia.org/wiki/Object" title="Object">Objects</a> implementing <a href="http://en.wikipedia.org/wiki/ArrayAccess" class="mw-redirect" title="ArrayAccess">ArrayAccess</a> can be used with array syntax and <a href="http://en.wikipedia.org/wiki/Object" title="Object">objects</a> implementing <a href="http://en.wikipedia.org/wiki/Iterator" title="Iterator">Iterator</a> or <a href="http://en.wikipedia.org/wiki/IteratorAggregate" class="mw-redirect" title="IteratorAggregate">IteratorAggregate</a> can be used with the <span>foreach</span> language construct. There is no <a href="http://en.wikipedia.org/wiki/Virtual_table" class="mw-redirect" title="Virtual table">virtual table</a> feature in the engine, so <a href="http://en.wikipedia.org/wiki/Static_variable" title="Static variable">static variables</a> are bound with a name instead of a reference at compile time.</p>
<p>If the developer creates a copy of an object using the reserved word <i>clone</i>, the Zend engine will check if a <tt>__clone()</tt> method has been defined or not. If not, it will call a default <tt>__clone()</tt> which will copy the object's properties. If a <tt>__clone()</tt> 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 <a href="http://en.wiktionary.org/wiki/replica" class="extiw" title="wikt:replica">replica</a> of the source object and only override properties that need to be changed.</p>
<p><a name="Resources" id="Resources"></a></p>
<h2> <span class="mw-headline">Resources</span></h2>
<p>PHP includes <a href="http://en.wikipedia.org/wiki/List_of_PHP_libraries" title="List of PHP libraries">free and open source libraries</a> with the core build. PHP is a fundamentally <a href="http://en.wikipedia.org/wiki/Internet" title="Internet">Internet</a>-aware system with modules built in for accessing <a href="http://en.wikipedia.org/wiki/File_transfer_protocol" class="mw-redirect" title="File transfer protocol">FTP</a> servers, many database servers, embedded SQL libraries such as embedded <a href="http://en.wikipedia.org/wiki/MySQL" title="MySQL">MySQL</a> and <a href="http://en.wikipedia.org/wiki/SQLite" title="SQLite">SQLite</a>, <a href="http://en.wikipedia.org/wiki/Lightweight_Directory_Access_Protocol" title="Lightweight Directory Access Protocol">LDAP</a> servers, and others. Many functions familiar to C programmers such as those in the <tt><a href="http://en.wikipedia.org/wiki/Stdio.h" title="Stdio.h">stdio</a></tt> family are available in the standard PHP build. PHP has traditionally used features such as "<a href="http://en.wikipedia.org/wiki/Magic_quotes" title="Magic quotes">magic_quotes_gpc</a>" 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 <a href="http://en.wikipedia.org/wiki/SQL_injection" title="SQL injection">SQL injection</a> 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. 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.</p>
<p>PHP allows developers to write <a href="http://en.wikipedia.org/wiki/Extension_%28computing%29" class="mw-redirect" title="Extension (computing)">extensions</a> in <a href="http://en.wikipedia.org/wiki/C_%28programming_language%29" title="C (programming language)">C</a> 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 <a href="http://en.wikipedia.org/wiki/Windows_API" title="Windows API">Windows API</a>, process management on <a href="http://en.wikipedia.org/wiki/Unix-like" title="Unix-like">Unix-like</a> <a href="http://en.wikipedia.org/wiki/Operating_system" title="Operating system">operating systems</a>, multibyte strings (<a href="http://en.wikipedia.org/wiki/Unispan" title="Unispan">Unispan</a>), <a href="http://en.wikipedia.org/wiki/CURL" title="CURL">cURL</a>, and several popular <a href="http://en.wikipedia.org/wiki/Compression_formats" class="mw-redirect" title="Compression formats">compression formats</a>. Some more unusual features include integration with <a href="http://en.wikipedia.org/wiki/Internet_relay_chat" class="mw-redirect" title="Internet relay chat">Internet relay chat</a>, dynamic generation of images and <a href="http://en.wikipedia.org/wiki/Adobe_Flash" title="Adobe Flash">Adobe Flash</a> content, and even <a href="http://en.wikipedia.org/wiki/Speech_synthesis" title="Speech synthesis">speech synthesis</a>. The <a href="http://en.wikipedia.org/wiki/PHP_Extension_Community_Library" class="mw-redirect" title="PHP Extension Community Library">PHP Extension Community Library</a> (PECL) project is a repository for extensions to the PHP language.</p>
<p>As with many scripting languages, PHP scripts are normally kept as human-readable source code, even on production web servers. While this allows flexibility, releasing scripts in source form is undesirable for commercial software developers, and can raise issues with security of web servers; as an example, if a hacker acquires control of a server, database passwords may be quickly discovered, and undesirable changes to scripts may be made that remain undiscovered indefinitely. Various encoding tools are available for PHP to offer code protection.</p>
<p>span 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 <a href="http://en.wikipedia.org/wiki/Compiler" title="Compiler">compiler</a> is such that there are often opportunities for <a href="http://en.wikipedia.org/wiki/Optimization_%28computer_science%29" title="Optimization (computer science)">span optimization</a>, and an example of a code optimizer is the <a href="http://en.wikipedia.org/wiki/PHP_accelerator#Zend_Optimizer" title="PHP accelerator">Zend Optimizer</a> PHP extension.</p>
<p><a href="http://en.wikipedia.org/wiki/PHP_accelerator" title="PHP accelerator">PHP accelerators</a> can offer significant performance gains by <a href="http://en.wikipedia.org/wiki/Caching" class="mw-redirect" title="Caching">caching</a> the compiled form of a PHP script in <a href="http://en.wikipedia.org/wiki/Shared_memory" title="Shared memory">shared memory</a> to avoid the overhead of <a href="http://en.wikipedia.org/wiki/Parsing" title="Parsing">parsing</a> and <a href="http://en.wikipedia.org/wiki/Compiling" class="mw-redirect" title="Compiling">compiling</a> the code every time the script runs. They may also perform <a href="http://en.wikipedia.org/wiki/span_optimization" class="mw-redirect" title="span optimization">span optimization</a> to provide increased execution performance.</p>
<p class="attribution">This excerpt is adapted from <a href="http://en.wikipedia.org/wiki/PHP#Syntax">Wikipedia: PHP - syntax</a>. It is used here under a <a href="http://en.wikipedia.org/wiki/Wikipedia:Text_of_Creative_Commons_Attribution-ShareAlike_3.0_Unported_License">Creative Commons BY-SA 3.0</a> license.</p>
