PHP syntax and semantics

ThesyntaxandsemanticsofPHP,aprogramming language,form a set of rules that define how a PHP program can be written and interpreted.

Overview

Historically, the development of PHP has been somewhat haphazard. To counter this, the PHP Framework Interop Group (FIG) has created ThePHP Standards Recommendation(PSR) documents that have helped bring more standardization to the language since 2009.The modern coding standards are contained in PSR-1 (Basic Coding Standard)and PSR-2 (Coding Style Guide).

Keywords

Some keywords represent things that look like functions, some look like constants, but they are actually language constructs. It is forbidden to use any keywords as constants, class names, functions or methods. Using them as variable names is allowed, but it can be confusing.

• __halt_compiler()
• abstract
• and
• array()
• as
• break
• callable(as of PHP 5.4)
• case
• catch
• class
• clone
• const
• continue
• declare
• default
• die()
• do
• echo
• else
• elseif
• empty()
• enddeclare
• endfor
• endforeach
• endif
• endswitch
• endwhile
• eval()
• exit()
• extends
• final
• finally(as of PHP 5.5)
• fn(as of PHP 7.4)
• for
• foreach
• function
• global
• goto(as of PHP 5.3)
• if
• implements
• include
• include_once
• instanceof
• insteadof(as of PHP 5.4)
• interface
• isset()
• list()
• match(as of PHP 8.0)
• namespace(as of PHP 5.3)
• new
• or
• print
• private
• protected
• public
• require
• require_once
• return
• static
• switch
• throw
• trait(as of PHP 5.4)
• try
• unset()
• use
• var
• while
• xor
• yield(as of PHP 5.5)
• yield from(as of PHP 7.0)

Basic language constructs

PHP generally followsC syntax,with exceptions and enhancements for its main use inweb development,which makes heavy use ofstring manipulation.PHP variables must be prefixed by "$".This allows PHP to performstring interpolationindouble quotedstrings, wherebackslashis supported as anescape character.No escaping or interpolation is done on strings delimited bysingle quotes.PHP also supports a C-likesprintffunction. Code can be modularized into functions defined with keywordfunction.PHP supports an optionalobject orientedcoding style, with classes denoted by theclasskeyword. Functions defined inside classes are sometimes calledmethods.Control structuresinclude:if,while,do/while,for,foreach,andswitch.Statements are terminated by a semicolon, not line endings.

Delimiters

The PHP processor only parses code within itsdelimiters.Anything outside its delimiters is sent directly to the output and not parsed by PHP. The only open/close delimiters allowed by PSR-1are "<?php"and"?>"or<?=and?>.

The purpose of the delimiting tags is to separate PHP code from non-PHP data (mainly HTML). Although rare in practice, PHP will execute code embedded in any file passed to its interpreter, includingbinary filessuch as PDF or JPEG files, or in server log files.Everything outside the delimiters is ignored by the PHP parser and is passed through as output.

These recommended delimiters create correctly formedXHTMLand otherXMLdocuments.This may be helpful if the source code documents ever need to be processed in other ways during the life of the software.

If proper XML validation is not an issue, and a file contains only PHP code, it is preferable to omit the PHP closing (?>) tag at the end of the file.

Non-recommended tags

Other delimiters can be used on some servers, though most are no longer supported.Examples are:

• "<script language= "php" >"and"</script>"(removed in PHP7)
• Short opening tags (<?) (configured with theshort_open_tagini setting)
  • A special form of the<?tag is<?=,which automatically echos the next statement. Prior to PHP 5.4.0 this was also controlled withshort_open_tag,but is always available in later versions.
• ASPstyle tags (<%or<%=) (removed in PHP7)

Variables and comments

Variables are prefixed with adollar symboland atypedoes not need to be specified in advance. Unlike function and class names, variable names are case-sensitive. Both double-quoted ("") andheredocstrings allow the ability to embed a variable's value into the string.As in C, variables may becastto a specific type by prefixing the type in parentheses. PHP treatsnewlinesaswhitespace,in the manner of afree-form language.Theconcatenationoperator is.(dot). Array elements are accessed and set withsquare bracketsin bothassociative arraysand indexed arrays.Curly bracketscan be used to access array elements, but not to assign.

PHP has three types ofcomment syntax:/* */which serves as block comments, and//as well as#which are used for inline comments.Many examples use theprintfunction instead of theechofunction. Both functions are nearly identical; the major difference being thatprintis slower thanechobecause the former will return a status indicating if it was successful or not in addition to text to output, whereas the latter does not return a status and only returns the text for output.

Simplest program

Main article:"Hello, World!" program

The usual "Hello World"code example for PHP is:

<?php
echo"Hello World!\n";
?>

The example above outputs the following:

Hello World!

Instead of using<?phpand theechostatement, an optional "shortcut" is the use of<?=instead of<?phpwhich implicitly echoes data. For example:

<!DOCTYPE html>
<html>
<head>
<title>PHP "Hello, World!" program</title>
</head>
<body>
<p><?="Hello World!"?></p>
</body>
</html>

The above example also illustrates that text not contained within enclosing PHP tags will be directly output.

Operators

Main articles:Operator (computer programming)andExpression (computer science)

PHP supports:arithmetic operators,assignment operators,bitwise operators,comparison operators,error control operators, execution operators,increment/decrement operators,logical operators,string operators,array operators,conditional assignment operators.

Control structures

Main articles:Control flowandStatement (computer science)

Conditionals

Main article:Conditional (computer programming)

If... else statement

The syntax of a PHPif... else statementis as follows:

if(condition){
// statements;
}elseif(condition2){
// statements;
}else{
// statements;
}

For single statements, the brackets may be omitted and the if optionally condensed to a single line:

if(condition)dosomething();
elseif(condition2)dosomethingelse();
elsedoyetathirdthing();

Ternary conditional operator

Main article:Ternary conditional operator § PHP

$abs=$value>=0?$value:-$value;

/* Equivalent to */

if($value>=0){
$abs=$value;
}else{
$abs=-$value;
}

Elvis operator

Main article:Elvis operator

Since PHP 5.3 supportsElvis operator(?:) in which it is possible to omit the middle part of the ternary operator.

$c=$a?:$b;

/* Equivalent to */

$c=$a?$a:$b;

Null coalescing operator

Main article:Null coalescing operator § PHP

Since version 7.0 PHP also supportsNull coalescing operator(??).

$a=$b??$c;

/* Equivalent to */

$a=isset($b)?$b:$c;

Since version 7.4 PHP also supportsNull coalescing operatorwith the??=syntax.

$a??=$b;

/* Equivalent to */

$a=$a??$b;

Safe navigation operator

Main article:Safe navigation operator § PHP

Since version 8.0 PHP also supportsSafe navigation operator(?->).

$variable=$object?->method();

/* Equivalent to */

$variable=$object!==null?$object->method():null;

Switch statement

Main article:Switch statement § PHP

An example of the syntax of a PHPswitch statementis as follows:

switch(expr){
case0:
// statements;
break;
case1:
// statements;
break;
case2:
// statements;
break;
default:
// statements;
}

Note that unlike inC,values in case statement can be any type, not just integers.

Match expression

PHP 8 introduces thematchexpression.The match expression is conceptually similar to aswitchstatement and is more compact for some use cases. switchstatements are traditionally favored for simple value-based comparisons,matchstatements provide more flexibility and readability, particularly when using in complex conditions or patterns

echomatch(1){
0=>'Foo',
1=>'Bar',
2=>'Baz',
};
//> Bar

Loops

Main article:Control flow § Loops

For loop

Main articles:Control flow § Count-controlled loops,andFor loop § 1995: PHP

The PHP syntax of afor loopis as follows:

for(initialization;condition;afterthought){
// statements;
}

While loop

Main articles:Control flow § Condition-controlled loops,andWhile loop § PHP

The syntax for a PHPwhile loopis as follows:

while(condition){
// statements;
}

Do while loop

Main articles:Control flow § Condition-controlled loops,andDo while loop § PHP

The syntax for a PHPdo while loopis as follows:

do{
// statements;
}while(condition);

For each loop

Main articles:Control flow § Collection-controlled loops,andForeach loop § PHP

The syntax for a PHPfor eachloop is as follows:

foreach($setas$value){
// statements;
}

Alternative syntax for control structures

PHP offers an alternative syntax using colons rather than the standard curly-brace syntax (of "{...}"). This syntax affects the following control structures:if,while,for,foreach,andswitch.The syntax varies only slightly from the curly-brace syntax. In each case the opening brace ({) is replaced with a colon (:) and the close brace is replaced withendif;,endwhile;,endfor;,endforeach;,orendswitch;,respectively.Mixing syntax styles within the same control block is not supported. An example of the syntax for anif/elseifstatement is as follows:

if(condition):
// code here
elseif(condition):
// code here
else:
// code here
endif;

This style is sometimes called template syntax, as it is often found easier to read when combining PHP and HTML or JavaScript for conditional output:

<html>
<?phpif($day=='Thursday'):?>
<div>Tomorrow is Friday!</div>
<?phpelseif($day=='Friday'):?>
<div>TGIF</div>
<?phpelse:?>
<div>ugh</div>
<?phpendif;?>
</html>

Exception handling

Main articles:Exception handlingandException handling syntax § PHP

Runtime exception handling method in PHP is inherited from C++.

functioninv($x)
{
if($x==0){
thrownewException('Division by zero');
}
return1/$x;
}

try{
echoinv(2);// prints 0.5
echoinv(0);// throw an exception
echoinv(5);// will not run
}catch(Exception$e){
echo$e->getMessage();// prints Division by zero
}

// Continue execution
echo"Hello";// prints Hello

Data types

Scalar types

Main article:Primitive data type

PHP supports four scalar types:bool,int,float,string.

Boolean

Main article:Boolean data type

PHP has a nativeBooleantype, named "bool",similar to the native Boolean types inJavaandC++.Using the Boolean type conversion rules, non-zero values are interpreted astrueand zero asfalse,as inPerl. Both constantstrueandfalseare case-insensitive.

Integer

Main article:Integer (computer science)

PHP stores whole numbers in a platform-dependent range. This range is typically that of 32-bit or 64-bit signed integers.Integer variables can be assigned using decimal (positive and negative),octal,hexadecimal,andbinarynotations.

$a=1234;// decimal number
$b=0321;// octal number (equivalent to 209 decimal)
$c=0x1B;// hexadecimal number (equivalent to 27 decimal)
$d=0b11;// binary number (equivalent to 3 decimal)
$e=1_234_567;// decimal number (as of PHP 7.4.0)

Float

Main article:Floating-point arithmetic

Real numbersare also stored in a platform-specific range. They can be specified usingfloating pointnotation, or two forms ofscientific notation.

$a=1.234;
$b=1.2e3;// 1200
$c=7E-5;// 0.00007
$d=1_234.567;// as of PHP 7.4.0

String

Main articles:String (computer science),Escape sequences in C,String interpolation § PHP,andHere document § PHP

PHP supportsstrings,which can be used with single quotes, double quotes, nowdoc orheredocsyntax.

Double quoted strings support variable interpolation:

$age='23';

echo"John is$ageyears old ";// John is 23 years old

Curly braces syntax:

$f="sqrt";
$x=25;

echo"a$xc\n";// Warning: Undefined variable $xc
echo"a{$x}c\n";// prints a25c
echo"a${x}c\n";// also prints a25c

echo"$f($x) is{$f($x)}\n";// prints sqrt(25) is 5

Special types

Main article:Null pointer

PHP supports two special types:null,resource. Thenulldata type represents a variable that has no value. The only value in thenulldata type isNULL.TheNULLconstant is not case sensitive.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.

Compound types

Main article:Compound data type

PHP supports four compound types:array,object,callable,iterable.

Array

Main articles:Associative array,Hash table,andComparison of programming languages (associative array) § PHP

Arrays can contain mixed elements of any type, including resources, objects.Multi-dimensional arrays are created by assigning arrays as array elements. PHP has no true array type. PHP arrays are nativelysparseandassociative.Indexed arrays are simply hashes using integers as keys.

Indexed array:

$season=["Autumn","Winter","Spring","Summer"];
echo$season[2];// Spring

Associative array:

$salary=["Alex"=>34000,"Bill"=>43000,"Jim"=>28000];
echo$salary["Bill"];// 43000

Multidimensional array:

$mark=[
"Alex"=>[
"biology"=>73,
"history"=>85
],
"Jim"=>[
"biology"=>86,
"history"=>92
]
];

echo$mark["Jim"]["history"];// 92

Object

Main article:Object (computer science)

Theobjectdata type is a combination of variables, functions and data structures in theobject-oriented programmingparadigm.

classPerson
{
//...
}

$person=newPerson();

Callable

Main articles:Callable object § In PHP,andFunction object § In PHP

Since version 5.3 PHP hasfirst-class functionsthat can be used e.g. as an argument to another function.

functionrunner(callable$function,mixed...$args)
{
return$function(...$args);
}

$f=fn($x,$y)=>$x**$y;

functionsum(int|float...$args)
{
returnarray_sum($args);
}

echorunner(fn($x)=>$x**2,2);// prints 4
echorunner($f,2,3);// prints 8
echorunner('sum',1,2,3,4);// prints 10

Iterable

Main articles:Iterator § PHP,Iterator pattern § PHP,andGenerator (computer programming) § PHP

Iterabletype indicate that variable can be used withforeachloop.It can be anyarrayorgeneratoror object that implementing the special internalTraversableinterface.

functionprintSquares(iterable$data)
{
foreach($dataas$value){
echo($value**2)."";
}

echo"\n";
}

// array
$array=[1,2,3,4,5,6,7,8,9,10];

// generator
$generator=function():Generator{
for($i=1;$i<=10;$i++){
yield$i;
}
};

// object
$arrayIterator=newArrayIterator([1,2,3,4,5,6,7,8,9,10]);

printSquares($array);// 1 4 9 16 25 36 49 64 81 100
printSquares($generator());// 1 4 9 16 25 36 49 64 81 100
printSquares($arrayIterator);// 1 4 9 16 25 36 49 64 81 100

Union types

Main article:Union type § PHP

Union types were introduced in PHP 8.0

functionfoo(string|int$foo):string|int{}

Functions

Main articles:Function (computer programming),First-class function,andHigher-order function § PHP

PHP has hundreds of base functions and thousands more from extensions. Prior to PHP version 5.3.0, functions are notfirst-class functionsand can only be referenced by their name, whereas PHP 5.3.0 introduces closures.User-defined functions can be created at any time and without being prototyped.Functions can be defined inside code blocks, permitting a run-time decision as to whether or not a function should be defined. There is no concept of local functions. Function calls must use parentheses with the exception of zero argument class constructor functions called with the PHPnewoperator, where parentheses are optional.

An example function definition is the following:

functionhello($target='World')
{
echo"Hello$target!\n";
}

hello();// outputs "Hello World!"
hello('Wikipedia');// outputs "Hello Wikipedia!"

Function calls may be made via variables, where the value of a variable contains the name of the function to call. This is illustrated in the following example:

functionhello()
{
return'Hello';
}

functionworld()
{
return"World!";
}

$function1='hello';
$function2='world';

echo"{$function1()}{$function2()}";

A default value for parameters can be assigned in the function definition, but prior to PHP 8.0 did not supportnamed parametersor parameter skipping.Some core PHP developers have publicly expressed disappointment with this decision.Others have suggested workarounds for this limitation.

Named arguments

Main article:Named arguments

Named arguments were introduced in PHP 8.0

functionpower($base,$exp)
{
return$base**$exp;
}

// Using positional arguments:
echopower(2,3);// prints 8

// Using named arguments:
echopower(base:2,exp:3);// prints 8
echopower(exp:3,base:2);// prints 8

Type declaration

Specifying the types of function parameters and function return values has been supported since PHP 7.0.

Return type declaration:

functionsum($a,$b):float
{
return$a+$b;
}

var_dump(sum(1,2));// prints float(3)

Parameters typing:

functionsum(int$a,int$b)
{
return$a+$b;
}

var_dump(sum(1,2));// prints int(3)
var_dump(sum(1.6,2.3));// prints int(3)

Strict typing

Without strict typing enabled:

$f1=fn($a,$b):int=>$a+$b;
$f2=fn(int$a,int$b)=>$a+$b;

var_dump($f1(1.3,2.6));// prints int(3)
var_dump($f1(1,'2'));// prints int(3)
var_dump($f2(1.3,2.6));// prints int(3)
var_dump($f2(1,'2'));// prints int(3)

With strict typing enabled:

declare(strict_types=1);

$f1=fn($a,$b):int=>$a+$b;
$f2=fn(int$a,int$b)=>$a+$b;

var_dump($f1(1.3,2.6));// Fatal error: Return value must be of type int, float returned
var_dump($f1(1,'2'));// prints int(3)
var_dump($f2(1.3,2.6));// Fatal error: Argument #1 ($a) must be of type int, float given
var_dump($f2(1,'2'));// Fatal error: Argument #2 ($b) must be of type int, string given

Anonymous functions

Main article:Anonymous functions § PHP

PHP supports trueanonymous functionsas of version 5.3.In previous versions, PHP only supported quasi-anonymous functions through thecreate_function()function.

$x=3;
$func=function($z){return$z*2;};
echo$func($x);// prints 6

Since version 7.4 PHP also supportsarrow functionssyntax (=>).

$x=3;
$func=fn($z)=>$z*2;
echo$func($x);// prints 6

Closures

Main article:Closure (computer programming)

Сreating closures

$add=fn($x)=>fn($y)=>$y+$x;

/* Equivalent to */

$add=function($x){
returnfunction($y)use($x){
return$y+$x;
};
};

using

$f=$add(5);

echo$f(3);// prints 8
echo$add(2)(4);// prints 6

PHP

This section is an excerpt fromVariadic function § In PHP.[edit]

PHPdoes not care about types of variadic arguments unless the argument is typed.

functionsum(...$nums):int
{
returnarray_sum($nums);
}

echosum(1,2,3);// 6

And typed variadic arguments:

functionsum(int...$nums):int
{
returnarray_sum($nums);
}

echosum(1,'a',3);// TypeError: Argument 2 passed to sum() must be of the type int (since PHP 7.3)

Generators

Main articles:Generator (computer programming) § PHP,andLazy evaluation

Using generators, we can write code that uses foreach to iterate over a dataset without having to create an array in memory, which can result in memory overhead or significant processing time for generation.

Objects

Main article:Object-oriented programming

Basicobject-oriented programmingfunctionality 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 likeprimitive types.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 byhandle,and not by value. PHP 5 introduced private and protectedmember variablesand methods, along withabstract classesandfinal classesas well asabstract methodsandfinal methods.It also introduced a standard way of declaringconstructorsanddestructors,similar to that of other object-oriented languages such asC++,and a standardexception handlingmodel. Furthermore PHP 5 addedInterfacesand allows for multiple Interfaces to be implemented. There are special interfaces that allow objects to interact with the runtime system.ObjectsimplementingArrayAccesscan be used with array syntax andobjectsimplementingIteratororIteratorAggregatecan be used with the foreachlanguage construct.The static method andclass variablefeatures in Zend Engine 2 do not work the way some would expect. There is novirtual tablefeature in the engine, sostatic variablesare bound with a name instead of a reference at compile time.

This example shows how to define a class,Foo,that inherits from classBar.The methodmyStaticMethodis a public static method that can be called withFoo::myStaticMethod();.

classFooextendsBar
{
function__construct()
{
$doo="wah dee dee";
}

publicstaticfunctionmyStaticMethod()
{
$dee="dee dee dum";
}
}

If the developer creates a copy of an object using the reserved wordclone,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-valuereplicaof the source object and only override properties that need to be changed.

Traits

This section is an excerpt fromTrait (computer programming) § PHP.[edit]

This example uses a trait to enhance other classes:

// The template
traitTSingleton
{
privatestatic$_instance=null;

privatefunction__construct(){}// Must have private default constructor and be aware not to open it in the class

publicstaticfunctiongetInstance()
{
if(null===self::$_instance){
self::$_instance=newself();
}

returnself::$_instance;
}
}

classFrontController
{
useTSingleton;
}

// Can also be used in already extended classes
classWebSiteextendsSomeClass
{
useTSingleton;
}

This allows simulating aspects of multiple inheritance:

traitTBounding
{
public$x,$y,$width,$height;
}

traitTMoveable
{
publicfunctionmoveTo($x,$y)
{
//…
}
}

traitTResizeable
{
publicfunctionresize($newWidth,$newHeight)
{
//…
}
}

classRectangle
{
useTBounding,TMoveable,TResizeable;

publicfunctionfillColor($color)
{
//…
}
}

See also

• HTML(Hypertext Markup Language)
• Web template system