The Basicsclass
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:
^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$.
A class may contain its
own constants, variables
(called "properties"), and functions (called "methods").
Simple Class definition
var;
}
}
?>
]]>
The pseudo-variable $this is available when a
method is called from within an object
context. $this is a reference to the calling
object (usually the object to which the method belongs, but
possibly another object, if the method is called
statically from the context
of a secondary object).
Some examples of the <varname>$this</varname> pseudo-variable
foo();
// Note: the next line will issue a warning if E_STRICT is enabled.
A::foo();
$b = new B();
$b->bar();
// Note: the next line will issue a warning if E_STRICT is enabled.
B::bar();
?>
]]>
&example.outputs;
new
To create an instance of a class, the new keyword must
be used. An object will always be created unless the object has a
constructor defined that throws an
exception on error. Classes
should be defined before instantiation (and in some cases this is a
requirement).
If a string containing the name of a class is used with
new, a new instance of that class will be created. If
the class is in a namespace, its fully qualified name must be used when
doing this.
Creating an instance
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In the class context, it is possible to create a new object by
new self and new parent.
When assigning an already created instance of a class to a new variable, the new variable
will access the same instance as the object that was assigned. This
behaviour is the same when passing instances to a function. A copy
of an already created object can be made by
cloning it.
Object Assignment
var = '$assigned will have this value';
$instance = null; // $instance and $reference become null
var_dump($instance);
var_dump($reference);
var_dump($assigned);
?>
]]>
&example.outputs;
string(30) "$assigned will have this value"
}
]]>
PHP 5.3.0 introduced a couple of new ways to create instances of an
object:
Creating new objects
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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. Reffering 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.
Property access vs. method call
bar, PHP_EOL, $obj->bar(), PHP_EOL;
]]>
&example.outputs;
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.
Calling an anonymous function stored in a property
bar = function() {
return 42;
};
}
}
$obj = new Foo();
$func = $obj->bar;
echo $func(), PHP_EOL;
]]>
&example.outputs;
extends
A class can inherit the methods and properties of another class by
using the keyword extends in the class
declaration. It is not possible to extend multiple classes; a
class can only inherit from one base class.
The inherited 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
as final, that method
may not be overridden. It is possible to access the overridden
methods or static properties by referencing them
with parent::.
When overriding methods, the parameter signature should remain the same or
PHP will generate an E_STRICT level error. This does
not apply to the constructor, which allows overriding with different
parameters.
Simple Class Inheritance
displayVar();
?>
]]>
&example.outputs;
::class
Since PHP 5.5, the class keyword is also used for class
name resolution. You can get a string containing the fully qualified name
of the ClassName class by using
ClassName::class. This is particularly useful with
namespaced classes.
Class name resolution
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The class name resolution using ::class 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.