Class Constants

    It is possible to define constants on a per-class basis remaining the same and unchangeable. The default visibility of class constants is public.

    Note:

    Class constants can be redefined by a child class. As of PHP 8.1.0, class constants cannot be redefined by a child class if it is defined as final.

    It's also possible for interfaces to have constants. Look at the interface documentation for examples.

    It's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static).

    Note that class constants are allocated once per class, and not for each class instance.

    Example #1 Defining and using a constant

    <?php
    class MyClass
    {
    const     CONSTANT = 'constant value';

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

    echo     MyClass::CONSTANT . "\n";

    $classname = "MyClass";
    echo     $classname::CONSTANT . "\n";

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

    echo     $class::CONSTANT."\n";
    ?>

    The special ::class constant allows for fully qualified class name resolution at compile time, this is useful for namespaced classes:

    Example #2 Namespaced ::class example

    <?php
    namespace foo {
    class     bar {
    }

    echo     bar::class; // foo\bar
    }
    ?>

    Example #3 Class constant expression example

    <?php
    const ONE = 1;
    class     foo {
    const     TWO = ONE * 2;
    const     THREE = ONE + self::TWO;
    const     SENTENCE = 'The value of THREE is '.self::THREE;
    }
    ?>

    Example #4 Class constant visibility modifiers, as of PHP 7.1.0

    <?php
    class Foo {
    public const     BAR = 'bar';
    private const     BAZ = 'baz';
    }
    echo     Foo::BAR, PHP_EOL;
    echo     Foo::BAZ, PHP_EOL;
    ?>

    Output of the above example in PHP 7.1:

    bar

Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …

    Note:

    As of PHP 7.1.0 visibility modifiers are allowed for class constants.

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    User Contributed Notes 12 notes

    up
    206
    tmp dot 4 dot longoria at gmail dot com
    13 years ago
    it's possible to declare constant in base class, and override it in child, and access to correct value of the const from the static method is possible by 'get_called_class' method:
    <?php
    abstract class dbObject
    {
    const     TABLE_NAME='undefined';

    public static function     GetAll()
    {
    $c = get_called_class();
    return     "SELECT * FROM `".$c::TABLE_NAME."`";
    }
    }

    class     dbPerson extends dbObject
    {
    const     TABLE_NAME='persons';
    }

    class     dbAdmin extends dbPerson
    {
    const     TABLE_NAME='admins';
    }

    echo     dbPerson::GetAll()."<br>";//output: "SELECT * FROM `persons`"
    echo dbAdmin::GetAll()."<br>";//output: "SELECT * FROM `admins`"

    ?>
    up
    144
    kuzawinski dot marcin at gmail dot com
    9 years ago
    As of PHP 5.6 you can finally define constant using math expressions, like this one:

    <?php

    class MyTimer {
    const     SEC_PER_DAY = 60 * 60 * 24;
    }

    ?>

    Me happy :)
    up
    152
    anonymous
    14 years ago
    Most people miss the point in declaring constants and confuse then things by trying to declare things like functions or arrays as constants. What happens next is to try things that are more complicated then necessary and sometimes lead to bad coding practices. Let me explain...

    A constant is a name for a value (but it's NOT a variable), that usually will be replaced in the code while it gets COMPILED and NOT at runtime.

    So returned values from functions can't be used, because they will return a value only at runtime.

    Arrays can't be used, because they are data structures that exist at runtime.

    One main purpose of declaring a constant is usually using a value in your code, that you can replace easily in one place without looking for all the occurences. Another is, to avoid mistakes.

    Think about some examples written by some before me:

    1. const MY_ARR = "return array(\"A\", \"B\", \"C\", \"D\");";
    It was said, this would declare an array that can be used with eval. WRONG! This is just a string as constant, NOT an array. Does it make sense if it would be possible to declare an array as constant? Probably not. Instead declare the values of the array as constants and make an array variable.

    2. const magic_quotes = (bool)get_magic_quotes_gpc();
    This can't work, of course. And it doesn't make sense either. The function already returns the value, there is no purpose in declaring a constant for the same thing.

    3. Someone spoke about "dynamic" assignments to constants. What? There are no dynamic assignments to constants, runtime assignments work _only_ with variables. Let's take the proposed example:

    <?php
    /**
    * Constants that deal only with the database
    */
    class DbConstant extends aClassConstant {
    protected     $host = 'localhost';
    protected     $user = 'user';
    protected     $password = 'pass';
    protected     $database = 'db';
    protected     $time;
    function     __construct() {
    $this->time = time() + 1; // dynamic assignment
    }
    }
    ?>

    Those aren't constants, those are properties of the class. Something like "this->time = time()" would even totally defy the purpose of a constant. Constants are supposed to be just that, constant values, on every execution. They are not supposed to change every time a script runs or a class is instantiated.

    Conclusion: Don't try to reinvent constants as variables. If constants don't work, just use variables. Then you don't need to reinvent methods to achieve things for what is already there.
    up
    117
    delete dot this dot and dot dots dot gt at kani dot hu
    10 years ago
    I think it's useful if we draw some attention to late static binding here:
    <?php
    class A {
    const     MY_CONST = false;
    public function     my_const_self() {
    return     self::MY_CONST;
    }
    public function     my_const_static() {
    return static::    MY_CONST;
    }
    }

    class     B extends A {
    const     MY_CONST = true;
    }

    $b = new B();
    echo     $b->my_const_self ? 'yes' : 'no'; // output: no
    echo $b->my_const_static ? 'yes' : 'no'; // output: yes
    ?>
    up
    92
    Xiong Chiamiov
    10 years ago
    const can also be used directly in namespaces, a feature never explicitly stated in the documentation.

    <?php
    # foo.php
    namespace Foo;

    const     BAR = 1;
    ?>

    <?php
    # bar.php
    require 'foo.php';

    var_dump(Foo\BAR); // => int(1)
    ?>
    up
    48
    jimmmy dot chief at gmail dot com
    8 years ago
    Hi, i would like to point out difference between self::CONST and $this::CONST with extended class.
    Let us have class a:

    <?php
    class a {
    const     CONST_INT = 10;

    public function     getSelf(){
    return     self::CONST_INT;
    }

    public function     getThis(){
    return     $this::CONST_INT;
    }
    }
    ?>

    And class b (which extends a)

    <?php
    class b extends a {
    const     CONST_INT = 20;

    public function     getSelf(){
    return     parent::getSelf();
    }

    public function     getThis(){
    return     parent::getThis();
    }
    }
    ?>

    Both classes have same named constant CONST_INT.
    When child call method in parent class, there is different output between self and $this usage.

    <?php
    $b     = new b();

    print_r($b->getSelf()); //10
    print_r($b->getThis()); //20

    ?>
    up
    28
    nepomuk at nepda dot de
    8 years ago
    [Editor's note: that is already possible as of PHP 5.6.0.]

    Note, as of PHP7 it is possible to define class constants with an array.

    <?php
    class MyClass
    {
    const     ABC = array('A', 'B', 'C');
    const     A = '1';
    const     B = '2';
    const     C = '3';
    const     NUMBERS = array(
    self::A,
    self::B,
    self::C,
    );
    }
    var_dump(MyClass::ABC);
    var_dump(MyClass::NUMBERS);

    // Result:
    /*
    array(3) {
    [0]=>
    string(1) "A"
    [1]=>
    string(1) "B"
    [2]=>
    string(1) "C"
    }
    array(3) {
    [0]=>
    string(1) "1"
    [1]=>
    string(1) "2"
    [2]=>
    string(1) "3"
    }
    */
    ?>
    up
    2
    powtac at gmx dot de
    1 year ago
    Since it it is not mentioned in the description the following types can be set as a class constant: string, array, int, bool and probably float. But not objects.

    <?php

    class Test {
    const     arr = array();
    const     string = 'string';
    const     int = 99;
    const     bool = true;
    }

    var_dump(
    (new     Test())::arr,
    (new     Test())::string,
    (new     Test())::int,
    (new     Test())::bool
    );

    /* ouput for PHP 7.0.0+:

    array(0) {
    }
    string(6) "string"
    int(99)
    bool(true)
    up
    8
    wbcarts at juno dot com
    16 years ago
    Use CONST to set UPPER and LOWER LIMITS

    If you have code that accepts user input or you just need to make sure input is acceptable, you can use constants to set upper and lower limits. Note: a static function that enforces your limits is highly recommended... sniff the clamp() function below for a taste.

    <?php

    class Dimension
    {
    const     MIN = 0, MAX = 800;

    public     $width, $height;

    public function     __construct($w = 0, $h = 0){
    $this->width = self::clamp($w);
    $this->height = self::clamp($h);
    }

    public function     __toString(){
    return     "Dimension [width=$this->width, height=$this->height]";
    }

    protected static function     clamp($value){
    if(    $value < self::MIN) $value = self::MIN;
    if(    $value > self::MAX) $value = self::MAX;
    return     $value;
    }
    }

    echo (new     Dimension()) . '<br>';
    echo (new     Dimension(1500, 97)) . '<br>';
    echo (new     Dimension(14, -20)) . '<br>';
    echo (new     Dimension(240, 80)) . '<br>';

    ?>

    - - - - - - - -
    Dimension [width=0, height=0] - default size
    Dimension [width=800, height=97] - width has been clamped to MAX
    Dimension [width=14, height=0] - height has been clamped to MIN
    Dimension [width=240, height=80] - width and height unchanged
    - - - - - - - -

    Setting upper and lower limits on your classes also help your objects make sense. For example, it is not possible for the width or height of a Dimension to be negative. It is up to you to keep phoney input from corrupting your objects, and to avoid potential errors and exceptions in other parts of your code.
    up
    3
    Paul
    9 years ago
    Square or curly bracket syntax can normally be used to access a single byte (character) within a string. For example: $mystring[5]. However, please note that (for some reason) this syntax is not accepted for string class constants (at least, not in PHP 5.5.12).
    For example, the following code gives "PHP Parse error: syntax error, unexpected '[' in php shell code on line 6".
    <?php
    class SomeClass
    {
    const     SOME_STRING = '0123456790';
    public static function     ATest()
    {
    return     self::SOME_STRING[0];
    }
    }
    ?>
    It looks like you have to use a variable/class member instead.
    up
    2
    Nimja
    7 years ago
    Note that this magic constant DOES NOT load classes. And in fact can work on classes that do not exist.

    This means it does not mess with auto-loading.

    <?php
    $className     = \Foo\Bar::class;
    var_dump($className);
    var_dump(class_exists($className, false));
    ?>

    Will output:

    string(7) "Foo\Bar"
    bool(false)
    up
    0
    David Spector
    6 years ago
    The usual comma-separated syntax can be used to declare several constants:

    class STATE
    {
    const INIT=0, NAME_SEEN=1, ADDR_SEEN=2;
    }

    This shows the declaration of a set of enumeration literals suitable for use in a finite state machine loop. Reference such an enum by using syntax such as "STATE::INIT". Its actual type in this case will be integer.
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