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 270.
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 271
$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 272
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 273
$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 274
$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 275
$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 276
$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 277
$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 278
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 readonly modifier to every declared property, and prevent the creation of dynamic properties. 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 279
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }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] { }1 modifier, readonly classes cannot declare them either:
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }2
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }3
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }4
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }5
A readonly class can be extended 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] { }6 keyword must be used. An object will always be created unless the object has a constructor defined that throws an exception on error. Classes should be defined before instantiation [and in some cases this is a requirement].
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] { }6, a new instance of that class will be created. If the class is in a namespace, its fully qualified name must be used when doing this.
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] { }8
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }9
As of PHP 8.0.0, using
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }6 with arbitrary expressions is supported. This allows more complex instantiation if the expression produces a string. The expressions must be wrapped in parentheses.
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" }1 constant.
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }2
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }3
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }4
Output of the above example in PHP 8:
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }
In the class context, it is possible to create a new object by
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }5 and
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }6.
When assigning an already created instance of a class to a new variable, the new variable will access the same instance as the object that was assigned. This behaviour is the same when passing instances to a function. A copy of an already created object can be made by cloning it.
Example #5 Object Assignment
object[ClassA]#1 [0] { } object[ClassB]#1 [0] { } object[ClassC]#1 [0] { } object[ClassD]#1 [0] { }8
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }8
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }9
bool[true] bool[true] bool[true]0
bool[true] bool[true] bool[true]1
The above example will output:
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$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]2
bool[true] bool[true] bool[true]3
bool[true] bool[true] bool[true]4
bool[true] bool[true] bool[true]5
bool[true] bool[true] bool[true]6
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]7
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]8
bool[true] bool[true] bool[true]9
Extending class a default value0
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 value1
Extending class a default value2
Extending class a default value3
Extending class a default value4
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 value5 in the class declaration. It is not possible to extend multiple classes; a class can only inherit from one base class.
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 value6
Extending class a default value7
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 value8 level error is generated. A signature is compatible if it respects the variance rules, makes a mandatory parameter optional, and if any new parameters are optional. This is known as the Liskov Substitution Principle, or LSP for short. The constructor, and
Extending class a default value9 methods are exempt from these signature compatibility rules, and thus won't emit a fatal error in case of a signature mismatch.
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 130
Fatal error: Declaration of Extend::foo[] must be compatible with Base::foo[int $a = 5] in /in/evtlq on line 131
Fatal error: Declaration of Extend::foo[] must be compatible with Base::foo[int $a = 5] in /in/evtlq on line 132
Fatal error: Declaration of Extend::foo[] must be compatible with Base::foo[int $a = 5] in /in/evtlq on line 133
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 134
Fatal error: Declaration of Extend::foo[] must be compatible with Base::foo[int $a = 5] in /in/evtlq on line 135
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 134
Fatal error: Declaration of Extend::foo[] must be compatible with Base::foo[int $a = 5] in /in/evtlq on line 137
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 named arguments 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 138
Fatal error: Declaration of Extend::foo[] must be compatible with Base::foo[int $a = 5] in /in/evtlq on line 139
Fatal error: Declaration of Extend::foo[int $a] must be compatible with Base::foo[int $a = 5] in /in/qJXVC on line 130
Fatal error: Declaration of Extend::foo[int $a] must be compatible with Base::foo[int $a = 5] in /in/qJXVC on line 131
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 133 use
Fatal error: Declaration of Extend::foo[int $a] must be compatible with Base::foo[int $a = 5] in /in/qJXVC on line 134. This is particularly useful with namespaced classes.
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 135
Fatal error: Declaration of Extend::foo[int $a] must be compatible with Base::foo[int $a = 5] in /in/qJXVC on line 136
The above example will output:
Note:
The class name resolution using
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$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 $a = 5] in /in/qJXVC on line 138The above example will output:
As of PHP 8.0.0, the
NULL NULL object[SimpleClass]#1 [1] { ["var"]=> string[30] "$assigned will have this value" }1 constant may also be used on objects. This resolution happens at runtime, not compile time. Its effect is the same as calling get_class[] on the object.
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 140
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 141. The nullsafe operator works the same as property or method access as above, except that if the object being dereferenced is
Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14 Stack trace: #0 {main} thrown in /in/XaaeN on line 142 then
Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14 Stack trace: #0 {main} thrown in /in/XaaeN on line 142 will be returned rather than an exception thrown. If the dereference is part of a chain, the rest of the chain is skipped.
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 144
Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14 Stack trace: #0 {main} thrown in /in/XaaeN on line 145
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.