PHP's Object-oriented journey: a deep understanding of static variables and methods _php techniques

The static keyword declares that a property or method is related to a class, not a particular instance of a class, and therefore such a property or method is also called a class property or a class method.

If access control permission allows, you do not have to create the class object and use the class name plus two colons "::" to invoke it directly.

The static keyword can be used to modify variables, methods.

static properties and static methods in a class can be accessed directly without instantiating it.

Static properties and methods that can access only static properties and methods, and cannot have classes that access non-static properties and methods. Because static properties and methods are created, there may not be any instances of this class that can be invoked.

Static properties, in memory only one copy, shared for all instances.

Use self:: keyword to access static members of the current class.
static Property Common attributes

All instances of a class that share static properties in a class.

That is, even if there are multiple instances in memory, there is only one copy of the static property.

The following example sets a counter $count property, setting private and static adornments. In this way, the $count property is not directly accessible to the outside world. As a result of the program running, we also see that multiple instances are using the same static $count property.

Copy Code code as follows:

?
Class user{
private static $count = 0; Log the logon status of all users.
Public Function __construct () {
Self:: $count = self:: $count + 1;
}
Public Function GetCount () {
Return self:: $count;
}
Public Function __destruct () {
Self:: $count = self:: $count-1;
}
}
$user 1 = new user ();
$user 2 = new User ();
$user 3 = new User ();
echo "Now here have". $user 1->getcount (). "User";
echo "<br>";
Unset ($user 3);
echo "Now here have". $user 1->getcount (). "User";
?>

Program Run Result:
1
2

Now here have 3 user
Now here have 2 user jb51.net
Static properties are called directly

Static properties can be used directly without instantiating them and can be used directly when the class is not yet created.

The method used is the class name:: Static property name.

Copy Code code as follows:

?
Class math{
public static $PI = 3.14;

}
Find the area of a garden with a radius of 3.
$r = 3;
echo "radius is $r area is <br>";
echo Math:: $pi * $r * $R;

echo "<br><br>";
Here I think 3.14 is not accurate enough, I set it more accurate.
Math:: $pi = 3.141592653589793;
echo "radius is $r area is <br>";
echo Math:: $pi * $r * $R;
?>

Program Run Result:
1
2
3
4

The radius is 3 of the area is
28.26
The radius is 3 of the area is
28.2743338823

Class is not created, static properties can be used directly. When is the static property created in memory? There is no relevant information in PHP. Refer to the concepts in Java to explain that they should also be generic.

Static properties and methods that are created when a class is invoked. Class is called, meaning that the class is created or any static members in the class are invoked.
static method

Static methods do not require that the class being instantiated be used directly.

The way you use it is the class name:: Static method name.

Let's continue to write this math class for mathematical calculations. We designed a method to calculate the maximum value. Since it is a mathematical operation, we do not need to instantiate this class, if this method can be used to be more convenient.

We are just this class designed to demonstrate the static method. The max () function comparison value is provided in PHP.

Copy Code code as follows:

?
Class math{

public static function Max ($num 1, $num 2) {
Return $num 1 > $num 2? $num 1: $num 2;
}
}
$a = 99;
$b = 88;
echo "shows the maximum value in $ A and $ B is";
echo "<br>";
Echo Math::max ($a, $b);
echo "<br>"; echo "<br>"; echo "<br>";
$a = 99;
$b = 100;
echo "shows the maximum value in $ A and $ B is";
echo "<br>";
Echo Math::max ($a, $b);
?>

Program Run Result:

The maximum value shown in $ A and $ B is
99
The maximum value shown in $ A and $ B is
100
How static methods Call static methods

The first example, when a static method invokes another static method, uses the class name directly.

Copy Code code as follows:

?
The math class that implements the maximum comparison.
Class math{

public static function Max ($num 1, $num 2) {
Return $num 1 > $num 2? $num 1: $num 2;
}
public static function Max3 ($num 1, $num 2, $num 3) {
$num 1 = Math::max ($num 1, $num 2);
$num 2 = Math::max ($num 2, $num 3);
$num 1 = Math::max ($num 1, $num 2);
return $num 1;
}
}
$a = 99;
$b = 77;
$c = 88;
echo "Displays the maximum value of $a $b $c is";
echo "<br>";
Echo math::max3 ($a, $b, $c);
?>

Program Run Result:
1
2

The maximum value shown in 99 77 88 is
99

You can also use self:: To invoke other static methods in the current class. Recommendations

Copy Code code as follows:

?
The math class that implements the maximum comparison.
Class math{

public static function Max ($num 1, $num 2) {
Return $num 1 > $num 2? $num 1: $num 2;
}
public static function Max3 ($num 1, $num 2, $num 3) {
$num 1 = Self::max ($num 1, $num 2);
$num 2 = Self::max ($num 2, $num 3);
$num 1 = Self::max ($num 1, $num 2);
return $num 1;
}
}
$a = 99;
$b = 77;
$c = 88;
echo "Displays the maximum value of $a $b $c is";
echo "<br>";
Echo math::max3 ($a, $b, $c);
?>

Program Run Result:
1
2

The maximum value shown in 99 77 88 is
99
Static methods Call Static properties

Use class Name:: Static property names call static properties in this class.

Copy Code code as follows:

?
//
Class circle{
public static $PI = 3.14;

public static function Circleacreage ($r) {
return $r * $r * Circle:: $PI;
}
}
$r = 3;
The area of the echo "Radius $r circle is". Circle::circleacreage ($R);
?>

Program Run Result:
1

The area of the circle with a radius of 3 is 28.26.

Use self:: Invoke the static properties of this class. Recommendations

Copy Code code as follows:

?
//
Class circle{
public static $PI = 3.14;

public static function Circleacreage ($r) {
return $r * $r * self:: $PI;
}
}
$r = 3;
The area of the echo "Radius $r circle is". Circle::circleacreage ($R);
?>

Program Run Result:
1

The area of the circle with a radius of 3 is 28.26.
Static methods cannot call Non-static properties

A static method cannot invoke a Non-static property. Cannot use Self:: Invoke Non-static property.

Copy Code code as follows:

?
//
Class circle{
Public $pi = 3.14;

public static function Circleacreage ($r) {
return $r * $r * self::p i;
}
}
$r = 3;
The area of the echo "Radius $r circle is". Circle::circleacreage ($R);
?>

Program Run Result:
1

Fatal error:undefined class constant ' pi ' in e:phpprojectstest.php on line 7

You cannot use $this to get the value of a non-static property.

Copy Code code as follows:

?
//
Class circle{
Public $pi = 3.14;

public static function Circleacreage ($r) {
return $r * $r * $this->pi;
}
}
$r = 3;
The area of the echo "Radius $r circle is". Circle::circleacreage ($R);
?>

Program Run Result:
1

Fatal error:using $this When isn't in the object context into e:phpprojectstest.php on line 7
Static method calls Non-static methods

In PHP5, non-static methods cannot be invoked in static methods using the $this identity.

Copy Code code as follows:

.
//The math class that implements the maximum comparison.
Class math{  
    public Function Max ($num 1, $num 2) {
    & nbsp;   echo "bad<br>";      
         return $num 1 > $num 2? $num 1: $num 2;
   }
    public static function Max3 ($num 1, $num 2, $num 3) {
       $ NUM1 = $this->max ($num 1, $num 2);
       $num 2 = $this->max ($num 2, $num 3);
       $num 1 = $this->max ($num 1, $num 2);      
       return $num 1;
   }
}
$a = 99;
$b = 77;
$c = 188; The maximum value in the
Echo display $a   $b $c   is ";
echo "<br>";
Echo math::max3 ($a, $b, $c);
?>

Program Run Result:

The maximum value shown in 99 77 188 is
Fatal error:using $this When isn't in the object context into e:test.php on line 10

When there is a Non-static method in a class that is self:: called, the system automatically converts this method to a static method.

The following code is executed and has the result. Because the Max method is converted to a static method by the system.

Copy Code code as follows:

?
The math class that implements the maximum comparison.
Class math{
Public Function Max ($num 1, $num 2) {
Return $num 1 > $num 2? $num 1: $num 2;
}
public static function Max3 ($num 1, $num 2, $num 3) {
$num 1 = Self::max ($num 1, $num 2);
$num 2 = Self::max ($num 2, $num 3);
$num 1 = Self::max ($num 1, $num 2);
return $num 1;
}
}
$a = 99;
$b = 77;
$c = 188;
echo "Displays the maximum value of $a $b $c is";
echo "<br>";
Echo math::max3 ($a, $b, $c);
?>

Program Run Result:
1
2

The maximum value shown in 99 77 188 is
188

In the following example, we let the static method Max3 use self:: A non-static method Max was invoked, with the Non-static method Max calling Non-static properties $pi by $this.

In the run is reported the error, this error is the same as the previous example 3-1-9.php, this time is not static method Max reported a static method call Non-static property error.

This is proof that the Non-static method we have defined here is automatically converted to a static method by the system.

Copy Code code as follows:

.
//The math class that implements the maximum comparison.
Class math{
    public $pi = 3.14;

    Public Function Max ($num 1, $num 2) {
        echo Self:: $pi; //The call here does not seem to be problematic.
        return $num 1 > $num 2 $num 1: $num 2;
   }
    public static function Max3 ($num 1, $num 2, $num 3) {
       $ NUM1 = Self::max ($num 1, $num 2);
       $num 2 = Self::max ($num 2, $num 3);
       $num 1 = Self::max ($num 1, $num 2);      
       return $num 1;
   }
}
$a = 99;
$b = 77;
$c = 188; The maximum value in the
Echo display $a   $b $c   is ";
echo "<br>";
Echo math::max3 ($a, $b, $c);
?>

Program Run Result:
1
2

The maximum value shown in 99 77 188 is
Fatal error:access to undeclared static Property:math:: $pi into e:phpprojectstest.php on line 7