Introduction
PHP allows an identifier in a class to be defined to have a constant value, the one that remains unchanged on a per class basis.To differentiate rom a variable or property wthin class, name of constant is not prefixed with $ symbol and is defined with const qualifier.
Default visibility of a constant is public, although other modifiers may be used in definition. Value of a constant must be certain expression and not a variable, nor a function call/property. value of constant is accessed through class name using scope resolution operator. Inside a method though it can be referred to through self variable
Syntax
class SomeClass{
const CONSTANT = 'constant value';
}
echo SomeClass::CONSTANT;Constant names are case sensitive. Conventionally, name of constant is given in upper case
Class Constant example
This example shows how a Class Constant is defined and accessed
Example
<?php
class square{
const PI=M_PI;
var $side=5;
function area(){
$area=$this->side**2*self::PI;
return $area;
}
}
$s1=new square();
echo "PI=". square::PI . "\n";
echo "area=" . $s1->area();
?>Output
This will produce following result. −
PI=3.1415926535898 area=78.539816339745
Class Constant as expression
In this example, class constant is assigned an expression
Example
<?php
const X = 22;
const Y=7;
class square {
const PI=X/Y;
var $side=5;
function area(){
$area=$this->side**2*self::PI;
return $area;
}
}
$s1=new square();
echo "PI=". square::PI . "\n";
echo "area=" . $s1->area();
?>outer
This will produce following result. −
PI=3.1428571428571 area=78.571428571429
Class constant visibility modifiers
Example
<?php
class example {
const X=10;
private const Y=20;
}
$s1=new example();
echo "public=". example::X. "\n";
echo "private=" . $s1->Y ."\n";
echo "private=" . $example::Y ."\n";
?>Output
This will produce following result. −
1public=10 PHP Notice: Undefined property: example::$Y in line 11 private=PHP Fatal error: Uncaught Error: Cannot access private const example::Y