In case this saves anyone any time, I spent ages working out why the following didn't work:
class MyClass
{
private $foo = FALSE;
public function __construct()
{
$this->$foo = TRUE;
echo($this->$foo);
}
}
$bar = new MyClass();
giving "Fatal error: Cannot access empty property in ...test_class.php on line 8"
The subtle change of removing the $ before accesses of $foo fixes this:
class MyClass
{
private $foo = FALSE;
public function __construct()
{
$this->foo = TRUE;
echo($this->foo);
}
}
$bar = new MyClass();
I guess because it's treating $foo like a variable in the first example, so trying to call $this->FALSE (or something along those lines) which makes no sense. It's obvious once you've realised, but there aren't any examples of accessing on this page that show that.
属性
类的变量成员叫做属性，或者叫字段，在本文档统一称为属性。
属性开头至少使用一个修饰符（比如 访问控制（可见性）、静态（static）关键字或者自 PHP 8.1.0 起支持的 readonly），
除了 readonly 属性之外都是可选的，然后自 PHP 7.4
起可以跟一个类型声明，然后跟一个普通的变量声明来组成。属性中的变量可以初始化，但是初始化的值必须是 常量值。
注意:
另一种过时的声明类属性的方法是使用
var关键字，而不是使用修饰符。
注意: 没有声明 访问控制（可见性） 修饰符的属性将默认声明为
public。
在类的成员方法里面，可以用 ->（对象运算符）：$this->property（其中
property 是该属性名）这种方式来访问非静态属性。静态属性则是用
::（双冒号）：self::$property
来访问。更多静态属性与非静态属性的区别参见 静态（static）关键字。
当一个方法在类定义内部被调用时，有一个可用的伪变量 $this。$this 是一个到主叫对象的引用（通常是该方法所从属的对象，但如果是从第二个对象静态调用时也可能是另一个对象）。
示例 #1 属性声明
<?php
class SimpleClass
{
public $var1 = 'hello ' . 'world';
public $var2 = <<<EOD
hello world
EOD;
public $var3 = 1+2;
// 错误的属性声明
public $var4 = self::myStaticMethod();
public $var5 = $myVar;
// 正确的属性声明
public $var6 = myConstant;
public $var7 = array(true, false);
public $var8 = <<<'EOD'
hello world
EOD;
// 没有访问控制修饰符：
static $var9;
readonly int $var10;
}
?>注意:
更多关于类／对象的处理函数，请查看类／对象函数。
类型声明
从 PHP 7.4.0 开始，属性定义可以包含类型声明，但 callable 除外。
示例 #2 类型声明的示例
<?php
class User
{
public int $id;
public ?string $name;
public function __construct(int $id, ?string $name)
{
$this->id = $id;
$this->name = $name;
}
}
$user = new User(1234, null);
var_dump($user->id);
var_dump($user->name);
?>以上示例会输出：
int(1234) NULL
类型属性必须在访问前初始化，否则会抛出 Error 。
示例 #3 访问属性
<?php
class Shape
{
public int $numberOfSides;
public string $name;
public function setNumberOfSides(int $numberOfSides): void
{
$this->numberOfSides = $numberOfSides;
}
public function setName(string $name): void
{
$this->name = $name;
}
public function getNumberOfSides(): int
{
return $this->numberOfSides;
}
public function getName(): string
{
return $this->name;
}
}
$triangle = new Shape();
$triangle->setName("triangle");
$triangle->setNumberofSides(3);
var_dump($triangle->getName());
var_dump($triangle->getNumberOfSides());
$circle = new Shape();
$circle->setName("circle");
var_dump($circle->getName());
var_dump($circle->getNumberOfSides());
?>以上示例会输出：
string(8) "triangle" int(3) string(6) "circle" Fatal error: Uncaught Error: Typed property Shape::$numberOfSides must not be accessed before initialization
只读属性
自 PHP 8.1.0 起，可以使用 readonly 修饰符声明属性，防止初始化后修改属性。
示例 #4 只读属性示例
<?php
class Test {
public readonly string $prop;
public function __construct(string $prop) {
// 初始化正常。
$this->prop = $prop;
}
}
$test = new Test("foobar");
// 读取正常。
var_dump($test->prop); // string(6) "foobar"
// 再赋值异常。分配的值是否相同并不重要。
$test->prop = "foobar";
// Error: Cannot modify readonly property Test::$prop
?>注意:
注意:
不支持对静态属性只读。
只读属性只能初始化一次，并且只能从声明它的作用域内初始化。对属性的任何赋值和修改都会导致 Error 异常。
示例 #5 初始化只读属性异常
<?php
class Test1 {
public readonly string $prop;
}
$test1 = new Test1;
// 私有作用域之外异常初始化。
$test1->prop = "foobar";
// Error: Cannot initialize readonly property Test1::$prop from global scope
?>注意:
禁止在只读属性上指定默认值，因为具有默认值的只读属性等同于常量，因此不是特别有用。
<?php
class Test {
// Fatal error: Readonly property Test::$prop cannot have default value
public readonly int $prop = 42;
}
?>
注意:
只读属性一旦初始化就不能 unset()。但可以在初始化之前从声明属性的作用域中取消只读属性。
修改不一定是简单的赋值，以下所有行为也会导致 Error 异常：
<?php
class Test {
public function __construct(
public readonly int $i = 0,
public readonly array $ary = [],
) {}
}
$test = new Test;
$test->i += 1;
$test->i++;
++$test->i;
$test->ary[] = 1;
$test->ary[0][] = 1;
$ref =& $test->i;
$test->i =& $ref;
byRef($test->i);
foreach ($test as &$prop);
?>然而，只读属性并不会妨碍内部可变性。存储在只读属性中的对象（或资源）仍然可以在内部修改：
<?php
class Test {
public function __construct(public readonly object $obj) {}
}
$test = new Test(new stdClass);
// 内部可变正常。
$test->obj->foo = 1;
// 赋值异常。
$test->obj = new stdClass;
?>
add a note
User Contributed Notes 5 notes
Anonymous ¶
12 years ago
anca at techliminal dot com ¶
9 years ago
You can access property names with dashes in them (for example, because you converted an XML file to an object) in the following way:
<?php
$ref = new StdClass();
$ref->{'ref-type'} = 'Journal Article';
var_dump($ref);
?>
Anonymous ¶
13 years ago
$this can be cast to array. But when doing so, it prefixes the property names/new array keys with certain data depending on the property classification. Public property names are not changed. Protected properties are prefixed with a space-padded '*'. Private properties are prefixed with the space-padded class name...
<?php
class test
{
public $var1 = 1;
protected $var2 = 2;
private $var3 = 3;
static $var4 = 4;
public function toArray()
{
return (array) $this;
}
}
$t = new test;
print_r($t->toArray());
/* outputs:
Array
(
[var1] => 1
[ * var2] => 2
[ test var3] => 3
)
*/
?>
This is documented behavior when converting any object to an array (see </language.types.array.php#language.types.array.casting> PHP manual page). All properties regardless of visibility will be shown when casting an object to array (with exceptions of a few built-in objects).
To get an array with all property names unaltered, use the 'get_object_vars($this)' function in any method within class scope to retrieve an array of all properties regardless of external visibility, or 'get_object_vars($object)' outside class scope to retrieve an array of only public properties (see: </function.get-object-vars.php> PHP manual page).
kchlin dot lxy at gmail dot com ¶
2 years ago
From PHP 8.1
It's easy to create DTO object with readonly properties and promoting constructor
which easy to pack into a compact string and covert back to a object.
<?php
# Conversion functions.
# Pack object into a compack JSON string.
function cvtObjectToJson( object $poObject ): string
{
return json_encode( array_values( get_object_vars( $poObject )));
}
# Unpack object from a JSON string.
function cvtJsonToObject( string $psClass, string $psString ): object
{
return new $psClass( ...json_decode( $psString ));
}
# DTO example class.
final class exampleDto
{
final public function __construct(
public readonly int $piInt,
public readonly ?int $pnNull,
public readonly float $pfFloat,
public readonly string $psString,
public readonly array $paArray,
public readonly object $poObject,
){}
}
# Example with export only public properties of given object.
$exampleDtoO = new exampleDto( 1, null, .3, 'string 4', [], new stdClass() );
$stringJson = cvtObjectToJson( $exampleDtoO );// [1,null,0.3,"string 4",[],{}]
$objectO = cvtJsonToObject( exampleDto::class, $stringJson );
# Check and var_dump variables.
echo $exampleDtoO == $objectO
? 'Objects equal, but not identical.'. PHP_EOL
: 'Objects not equal neither identical.'. PHP_EOL
;
var_dump($exampleDtoO, $stringJson, $objectO);
# Output
/*
Objects equal, but not identical
object(exampleDto)#6 (6) {
["piInt"]=>
int(1)
["pnNull"]=>
NULL
["pfFloat"]=>
float(0.3)
["psString"]=>
string(8) "string 4"
["paArray"]=>
array(0) {
}
["poObject"]=>
object(stdClass)#7 (0) {
}
}
string(29) "[1,null,0.3,"string 4",[],{}]"
object(exampleDto)#8 (6) {
["piInt"]=>
int(1)
["pnNull"]=>
NULL
["pfFloat"]=>
float(0.3)
["psString"]=>
string(8) "string 4"
["paArray"]=>
array(0) {
}
["poObject"]=>
object(stdClass)#9 (0) {
}
}
*/
zzzzBov ¶
14 years ago
Do not confuse php's version of properties with properties in other languages (C++ for example). In php, properties are the same as attributes, simple variables without functionality. They should be called attributes, not properties.
Properties have implicit accessor and mutator functionality. I've created an abstract class that allows implicit property functionality.
<?php
abstract class PropertyObject
{
public function __get($name)
{
if (method_exists($this, ($method = 'get_'.$name)))
{
return $this->$method();
}
else return;
}
public function __isset($name)
{
if (method_exists($this, ($method = 'isset_'.$name)))
{
return $this->$method();
}
else return;
}
public function __set($name, $value)
{
if (method_exists($this, ($method = 'set_'.$name)))
{
$this->$method($value);
}
}
public function __unset($name)
{
if (method_exists($this, ($method = 'unset_'.$name)))
{
$this->$method();
}
}
}
?>
after extending this class, you can create accessors and mutators that will be called automagically, using php's magic methods, when the corresponding property is accessed.
