PHP Classes

A class is like a blueprint, Which contains a set of various types of data & methods. Like which data should be accessible outside of class and which data should be accessible only inside of class. 

Example of a simple class:

<?php

class SimpleClass
{
    // property declaration
    public $var = 'a default value';

    // method declaration
    public function displayVar()
    {
        echo $this->var;
    }
}

?>

Class properties

The variables, constants & functions of the class are called as a class’s properties. These properties are defined by visibility keywords public, private & protected.

This declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile-time and must not depend on run-time information in order to be evaluated.
 

To access these properties inside of class, use pseudo-variable {$this} to accesses these properties. This pseudo-variable is predefined in every class, there is no need to declare pseudo-variable.
 

Example:

$this→property;
// this object may be a variables
// this object may be a constant
// this may be a function

How to declare properties

<?php

class SimpleClass
{
   // valid as of PHP 5.6.0:
   public $var1 = 'hello ' . 'world';

   // valid as of PHP 5.3.0:
   public $var2 = <<<EOD
   hello world
   EOD;

   // valid as of PHP 5.6.0:
   public $var3 = 1+2;

   // invalid property declarations:
   public $var4 = self::myStaticMethod();
   public $var5 = $myVpaar;

   // valid property declarations:
   public $var6 = myConstant;
   public $var7 = array(true, false);

   // valid as of PHP 5.3.0:
   public $var8 = <<<'EOD'
   hello world
   EOD;
}

?>

Class constants

PHP oops supports constants in class, there are you can declare and use constant’s value per-class.

The value must be a constant expression, not a variable, a property, or a function call. It's also possible for interfaces to have constants.

How to declare constant

<?php

class MyClass
{
    const CONSTANT = 'constant value';

    function showConstant() {
        echo  self::CONSTANT . "\n";
    }
}

echo MyClass::CONSTANT . "\n";

$classname = "MyClass";
echo $classname::CONSTANT . "\n"; // As of PHP 5.3.0

$class = new MyClass();
$class->showConstant();

echo $class::CONSTANT."\n"; // As of PHP 5.3.0

?>

Class constant visibility modifiers

<?php

class Foo {
    // As of PHP 7.1.0
    public const BAR = 'bar';
    private const BAZ = 'baz';
}
echo Foo::BAR, PHP_EOL;
echo Foo::BAZ, PHP_EOL;

?>

Output

• BAR constant will successfully be called.

• BAZ constant will give a fatal error because this constant visibility is private.