php class variable

PHP Class Variable (Declaration With Examples)

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What are Properties?

According to PHP documentation, member variables in a class are known as properties.

They may also be referred to as fields, but for the sake of this discussion, the term properties shall be used.

As of PHP 7.4, they are defined with at least one modifier (such as visibility, static keyword, or, as of PHP 8.1.0, read-only), optionally (except for read-only properties), followed by a type declaration, then a normal variable declaration.

This declaration may contain an initialization, but it must consist of a constant value.

Note: Using the var keyword instead of a modifier to declare class properties is deprecated.

Note: A property declared without a visibility modifier is public by default.

Non-static properties may be accessed within class methods by using -> (Object Operator): $this->property (where property is the property’s name).

The :: (Double Colon) is utilized to access static properties: self::$property. For additional details on the distinction between static and non-static attributes, see Static Keyword.

The artificial variable when a class method is invoked from within an object context, $this is accessible within the method. $this is the value of the object that is calling.

Example of Property Declaration

<?php
class SimpleClass
{
   public $var1 = 'hello ' . 'world';
   public $var2 = <<<EOD
hello world
EOD;
   public $var3 = 1+2;
   // invalid property declarations:
   public $var4 = self::myStaticMethod();
   public $var5 = $myVar;

   // valid property declarations:
   public $var6 = myConstant;
   public $var7 = [true, false];

   public $var8 = <<<'EOD'
hello world
EOD;

   // Without visibility modifier:
   static $var9;
   readonly int $var10;
}
?>

Note: There are several functions to handle classes and objects. See the reference for Class/Object Functions.

Type Declarations

With the exception of callable, property definitions can include Type declarations as of PHP 7.4.0.

Example of Typed Properties in PHP Class Variables

<?php

class User
{
    public int $id;
    public ?string $name;

    public function __construct(int $id, ?string $name)
    {
        $this->id = $id;
        $this->name = $name;
    }
}

$user = new User(2468, "Prince");

var_dump($user->id);
var_dump($user->name);

?>

The above example will output:

int(2468) string(6) "Prince"

If typed properties are not initialized before access, an Error is thrown.

Example of Accessing Properties in PHP Class Variables

<?php
    class Shape {
        public int $numberOfSides;
        public string $name;
        
        public function setNumberOfSides(int $numberOfSides): void {
            $this->numberOfSides = $numberOfSides;
        }

        public function setName(string $name): void {
            $this->name = $name;
        }

        public function getNumberOfSides(): int {
            return $this->numberOfSides;
        }
        
        public function getName(): string {
            return $this->name;
        }
        
    }
    
    $triangle = new Shape();
    $triangle->setName("square");
    $triangle->setNumberofSides(4);
    
    var_dump($triangle->getName());
    var_dump($triangle->getNumberOfSides());
    
    
    $circle = new Shape();
    $circle->setName("circle");
    
    var_dump($circle->getName());
    var_dump($circle->getNumberOfSides());
?>

The above example will output:

string(6) "square" int(4) string(6) "circle"
Fatal error:  Uncaught Error: Typed property Shape::$numberOfSides must not be accessed before initialization

Readonly Properties in PHP Class Variables

Since PHP 8.1.0, a property can be declared with the readonly modifier, which prohibits the property from being modified after initialization.

Example of Readonly Properties in PHP Class Variables

<?php
    class Test {
        public readonly string $prop;

        public function __construct(string $prop) {
            // This is a Legal initialization
            $this->prop = $prop;
        }
    }

    $test = new Test("sample text");
    // This is a Legal read
    var_dump($test->prop); // output would be string(11) "sampletext"

    // Illegal reassignment. It does not matter that the assigned value is the same.
    $test->prop = "sampletext";
    // Error: Cannot modify readonly property Test::$prop
?>

Note: The readonly modifier is only applicable to typed properties. Using the mixed type, a read-only property without type constraints can be generated.

Note: Not supported are readonly static properties.

A readonly property can only be initialized once, and only inside the declaration’s scope.

Any additional assignment or modification of the property will result in an exception of type Error.

Example of Illegal Initialization of Readonly Properties in PHP Class Initialize Variables

<?php
    class Test1 {
        public readonly string $prop;
    }

    $test1 = new Test1;
    // Illegal initialization outside of private scope.
    $test1->prop = "sample text";
    // Error: Cannot initialize readonly property Test1::$prop from global scope
?>

The above example will output:

Fatal error:  Uncaught Error: Cannot initialize readonly property Test1::$prop from global scope

Note: A readonly property with a default value is essentially the same as a constant and therefore not particularly useful.

<?php
    class Test {
        // Fatal error: Readonly property Test::$prop cannot have default value
        public readonly int $prop = 42;
    }
?>

The above example will output:

Fatal error:  Readonly property Test::$prop cannot have default value

Note: Once initialized, readonly properties cannot be unset using the unset() method.

However, prior to initialization, it is possible to unset a readonly property from the scope where it was declared.

Modifications are not always simple assignments; the following will also generate an Error exception:

<?php
    class Test {
        public function __construct(
            public readonly int $i = 0,
            public readonly array $ary = [],
        ) {}
    }

    $test = new Test;
    $test->i += 1;
    $test->i++;
    ++$test->i;
    $test->ary[] = 1;
    $test->ary[0][] = 1;
    $ref =& $test->i;
    $test->i =& $ref;
    byRef($test->i);
    foreach ($test as &$prop);
?>

However, readonly features do not prohibit the possibility of inner modification.

Objects (or resources) whose properties are set to readonly can nonetheless be updated internally:

<?php
    class Test {
        public function __construct(public readonly object $obj) {}
    }

    $test = new Test(new stdClass);
    // Legal interior mutation.
    $test->obj->bar = 1;
    // Illegal reassignment.
    $test->obj = new stdClass;
?>

Summary

In summary, we have learned about the PHP Class Variable through examples.

I hope you can now apply this information to your PHP project apps.

Lastly, if you want to learn more about PHP Class Variable, please leave a comment below. We’ll be happy to hear it!

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