PHP Access Modifiers

September 18, 2013

PHP access modifiers are used to set access rights with PHP classes and their members that is the functions and variables defined within the class scope. In PHP, there are some keywords representing these access modifiers. These keywords will be added with PHP classes and its members.

PHP keywords as Access Control Modifiers

Now, let us have a look into the following list to know about the possible PHP keywords used as access modifiers.

  1. public – class or its members defined with this access modifier will be publicly accessible from anywhere, even from outside the scope of the class.
  2. private – class members with this keyword will be accessed within the class itself. It protects members from outside class access with the reference of the class instance.
  3. protected – same as private, except by allowing sub classes to access protected super class members.
  4. abstract – This keyword can be used only for PHP classes and its functions. For containing abstract functions, a PHP class should be an abstract class.
  5. final – It prevents sub classes to override super class members defined with final keyword.

php_access_modifiers

Access modifiers Usage with PHP Classes, Class Members

Based on the possibility of applying the above list of PHP access modifiers, we can classify them. The following tabular column specify which keyword could be applied where in classes,functions or methods.

access modifier classes functions variable
public Not Applicable Applicable Applicable
private Not Applicable Applicable Applicable
protected Not Applicable Applicable Applicable
abstract Applicable Applicable Not Applicable
final Applicable Applicable Not Applicable

public Access Modifier

Before introducing access modifiers in PHP, all classes and its members are treated as public by default. Still, for PHP classes without specifying any access modifiers, then it will be treated as public.

If we specify public,private or protected for a PHP class, then it will cause parse error. For example, if we define a class as,

public class  {
...
...
}

Then, the following parse error will be displayed to the browser.

Parse error: syntax error, unexpected 'public' (T_PUBLIC) in ...

Similarly, class functions without any access modifiers, will be treated as public functions. But, it is good programming practice to specify those functions as public for better understanding.

Unlike PHP classes and functions, we need to specify access modifiers for PHP class variables explicitly. For example, the following code is invalid, which will cause PHP parse error.

class Toys {
$categories = array("puzzles","pull back","remote","soft");
...
}

And, the valid code for defining classes with public variables are follows.

class Toys {
public $categories = array("puzzles","pull back","remote","soft");
...
}

private Access Modifier

We can state this keyword only for class variables and function, but not for class itself, as like as public modifier. PHP class members defined as private cannot be accessed directly by using its instance. For example,

class Toys {
private $categories = array("puzzles","pull back","remote","soft");

public function getToysCategories() {
return $this->categories;
}
}
$objToys = new Toys();
print "<pre>";
print_r($objToys->categories); // invalid
print_r($objToys->getToysCategories());
print "</pre>";

In the above program, Toys class contains an private variable and a public function. Accessing this private variable by using Toys class instance with the line,

print_r($objToys->categories); // invalid

will cause PHP fatal error as,

Fatal error:  Cannot access private property Toys::$categories in ...

But we can access class private variables via any public function of that class. In the above program we are getting the elements of $categories array variables via getToysCategories() function defined as public.

Note that, all members of a class can be access within the class scope by using $this variable.

protected Access Modifier

This keyword is used in a PHP program which is using PHP inheritance. And, it is used to prevent access for PHP classes and its members from anywhere, except from inside the class itself or from inside its subclasses.

With this keyword, PHP classes and its members functions cannot be access directly from outside the classes and their subclasses, with the references of their instance.

Consider the following PHP program.

<?php
class Toys {
protected $categories = array("puzzles","pull back","remote","soft");
protected $toys = array(array("name"=>"Mechanical Cars","category"=>"pull back"),
array("name"=>"Jigsaw","category"=>"puzzles"),
array("name"=>"HiTech Cars","category"=>"remote"),
array("name"=>"Teddy Bears","category"=>"soft"),
array("name"=>"Baby pillow","category"=>"soft"),
array("name"=>"Chinese Checker","category"=>"puzzles"),
array("name"=>"Electronic Toys","category"=>"remote"));
protected function getToys() {
for($i=0;$i<count($this->toys);$i++) {
$toys_list[] = $this->toys[$i]["name"];
}
return $toys_list;
}
protected function getToysByCategory($category) {
for($i=0;$i<count($this->toys);$i++) {
if($this->toys[$i]["category"]==$category) 
$toys_list[] = $this->toys[$i]["name"];
}
return $toys_list;
}
}

class SoftToys extends Toys {
protected $category = "soft";
function getToys() {
return $this->getToysByCategory($this->category);
}
}

$objToys = new Toys();
$objSoftToys = new SoftToys();
print "<pre>";
/**Invalid
print_r($objToys->categories);
print_r($objSoftToys->categories);
print_r($objToys->getToys());*/
print_r($objSoftToys->getToys());
print "</pre>";
?>

In this program, we can see some of the statements are commented. These are invalid attempts to access protected members from outside Toys class.

We can access these members, as we have done with private member access by using the public function of a class.

abstract Access Modifier

We can define a PHP class or its function as abstract, but, abstract is not applicable for class variables. For having at least one abstract function in a PHP class, then that class should be an abstract class.

We cannot access PHP abstract class members with its instance. Because, PHP restricts instantiating abstract class and cause the following error message while creating such instance.

Fatal error: Cannot instantiate abstract class ...

But we can inherit an abstract class. Simply to say, PHP abstract classes are behaving like PHP interfaces. For an abstract class, we cannot contain any abstract function definition. Rather, we should just declare a function. For defining it, we should inherit this abstract class for a non-abstract subclass to have function definition.

Following code is an example for PHP abstract modifier.

abstract class Toys {
public $categories = array("puzzles","pull back","remote","soft");
public $toys = array(array("name"=>"Mechanical Cars","category"=>"pull back"),
array("name"=>"Jigsaw","category"=>"puzzles"),
array("name"=>"HiTech Cars","category"=>"remote"),
array("name"=>"Teddy Bears","category"=>"soft"),
array("name"=>"Baby pillow","category"=>"soft"),
array("name"=>"Chinese Checker","category"=>"puzzles"),
array("name"=>"Electronic Toys","category"=>"remote"));
abstract function getToys();

function getToysByCategory($category) {
for($i=0;$i<count($this->toys);$i++) {
if($this->toys[$i]["category"]==$category) 
$toys_list[] = $this->toys[$i]["name"];
}
return $toys_list;
}
}

class SoftToys extends Toys {
protected $category = "soft";
function getToys() {
return $this->getToysByCategory($this->category);
}
}

$objSoftToys = new SoftToys();
print "<pre>";
print_r($objSoftToys->getToys());
print "</pre>";

final Access Modifier

With PHP final access modifier classes are prevented from inheriting them. If we attempt to derive sub classes from PHP final classes, then the following fatal error will occur to stop program execution.

Fatal error: Class SoftToys may not inherit from final class ...

PHP final keyword should be used for PHP classes and functions, but not for class properties or variables. PHP functions with final keyword will not be overridden.

Download PHP Access Modifiers Source Code

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