Hướng dẫn dùng dn-attributes-failure trong PHP

Basic class definitions begin with the keyword class, followed by a class name, followed by a pair of curly braces which enclose the definitions of the properties and methods belonging to the class.

The class name can be any valid label, provided it is not a PHP reserved word. A valid class name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus:

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

A class may contain its own constants, variables (called "properties"), and functions (called "methods").

Example #1 Simple Class definition

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

The pseudo-variable $this is available when a method is called from within an object context. $this is the value of the calling object.

Warning

Calling a non-static method statically throws an Error. Prior to PHP 8.0.0, this would generate a deprecation notice, and $this would be undefined.

Example #2 Some examples of the $this pseudo-variable

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

Output of the above example in PHP 7:

$this is defined (A)

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 27
$this is not defined.

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 20
$this is not defined.

Deprecated: Non-static method B::bar() should not be called statically in %s  on line 32

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 20
$this is not defined.

Output of the above example in PHP 8:

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

Readonly classes

As of PHP 8.2.0, a class can be marked with the readonly modifier. Marking a class as readonly will add the to every declared property, and prevent the creation of . Moreover, it is impossible to add support for them by using the AllowDynamicProperties attribute. Attempting to do so will trigger a compile-time error.

$this is defined (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

As neither untyped, nor static properties can be marked with the

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

A readonly class can be if, and only if, the child class is also a readonly class.

new

To create an instance of a class, the

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

If a string containing the name of a class is used with

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

Note:

If there are no arguments to be passed to the class's constructor, parentheses after the class name may be omitted.

Example #3 Creating an instance

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

As of PHP 8.0.0, using

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

Example #4 Creating an instance using an arbitrary expression

In the given example we show multiple examples of valid arbitrary expressions that produce a class name. This shows a call to a function, string concatenation, and the

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

Output of the above example in PHP 8:

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

In the class context, it is possible to create a new object by

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

When assigning an already created instance of a class to a new variable, the new variable will access the same instance as the object that was assigned. This behaviour is the same when passing instances to a function. A copy of an already created object can be made by cloning it.

Example #5 Object Assignment

object(ClassA)#1 (0) {
}
object(ClassB)#1 (0) {
}
object(ClassC)#1 (0) {
}
object(ClassD)#1 (0) {
}

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

bool(true)
bool(true)
bool(true)

bool(true)
bool(true)
bool(true)

The above example will output:

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

It's possible to create instances of an object in a couple of ways:

Example #6 Creating new objects

bool(true)
bool(true)
bool(true)

bool(true)
bool(true)
bool(true)

bool(true)
bool(true)
bool(true)

bool(true)
bool(true)
bool(true)

bool(true)
bool(true)
bool(true)

The above example will output:

bool(true)
bool(true)
bool(true)

It is possible to access a member of a newly created object in a single expression:

Example #7 Access member of newly created object

bool(true)
bool(true)
bool(true)

The above example will output something similar to:

Note: Prior to PHP 7.1, the arguments are not evaluated if there is no constructor function defined.

Properties and methods

Class properties and methods live in separate "namespaces", so it is possible to have a property and a method with the same name. Referring to both a property and a method has the same notation, and whether a property will be accessed or a method will be called, solely depends on the context, i.e. whether the usage is a variable access or a function call.

Example #8 Property access vs. method call

bool(true)
bool(true)
bool(true)

bool(true)
bool(true)
bool(true)

Extending class
a default value

The above example will output:

That means that calling an anonymous function which has been assigned to a property is not directly possible. Instead the property has to be assigned to a variable first, for instance. It is possible to call such a property directly by enclosing it in parentheses.

Example #9 Calling an anonymous function stored in a property

Extending class
a default value

Extending class
a default value

Extending class
a default value

Extending class
a default value

The above example will output:

extends

A class can inherit the constants, methods, and properties of another class by using the keyword

Extending class
a default value

The inherited constants, methods, and properties can be overridden by redeclaring them with the same name defined in the parent class. However, if the parent class has defined a method or constant as final, they may not be overridden. It is possible to access the overridden methods or static properties by referencing them with parent::.

Note: As of PHP 8.1.0, constants may be declared as final.

Example #10 Simple Class Inheritance

Extending class
a default value

Extending class
a default value

The above example will output:

Extending class
a default value

Signature compatibility rules

When overriding a method, its signature must be compatible with the parent method. Otherwise, a fatal error is emitted, or, prior to PHP 8.0.0, an

Extending class
a default value

Extending class
a default value

Example #11 Compatible child methods

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

The above example will output:

The following examples demonstrate that a child method which removes a parameter, or makes an optional parameter mandatory, is not compatible with the parent method.

Example #12 Fatal error when a child method removes a parameter

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Output of the above example in PHP 8 is similar to:

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Example #13 Fatal error when a child method makes an optional parameter mandatory

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Output of the above example in PHP 8 is similar to:

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

Warning

Renaming a method's parameter in a child class is not a signature incompatibility. However, this is discouraged as it will result in a runtime Error if are used.

Example #14 Error when using named arguments and parameters were renamed in a child class

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

The above example will output something similar to:

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

::class

The class keyword is also used for class name resolution. To obtain the fully qualified name of a class

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

Example #15 Class name resolution

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

The above example will output:

Note:

The class name resolution using

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

1 is a compile time transformation. That means at the time the class name string is created no autoloading has happened yet. As a consequence, class names are expanded even if the class does not exist. No error is issued in that case.

Example #16 Missing class name resolution

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

8

The above example will output:

As of PHP 8.0.0, the

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

Example #17 Object name resolution

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

The above example will output:

Nullsafe methods and properties

As of PHP 8.0.0, properties and methods may also be accessed with the "nullsafe" operator instead:

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

The effect is similar to wrapping each access in an is_null() check first, but more compact.

Example #18 Nullsafe Operator

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

Note:

The nullsafe operator is best used when null is considered a valid and expected possible value for a property or method return. For indicating an error, a thrown exception is preferable.