PHP allows an identifier in a class to be defined as a “class constant” with a constant value, the one that remains unchanged on a per class basis. To differentiate from a variable or property within class, the name of the constant is not prefixed with the usual “$” symbol and is defined with the “const” qualifier. Note that a PHP program can also have a global constant created using the define() function.
The default visibility of a constant is public, although other modifiers may be used in the definition. The value of a constant must be an expression and not a variable, nor a function call/property. The value of a constant is accessed through the class name using the scope resolution operator. Inside a method though, it can be referred to through self variable.
class SomeClass {
const CONSTANT = ''constant value
}
echo SomeClass::CONSTANT;
Constant names are case sensitive. Conventionally, the names of constants are in upper case.
Example
This example shows how a Class Constant is defined and accessed −
<?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();
?>
It will produce the following output −
PI=3.1415926535898 area=78.539816339745
Class Constant as Expression
In this example, the class constant is assigned an expression −
<?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();
?>
It will produce the following output −
PI=3.1428571428571 area=78.571428571429
Class Constant Visibility Modifiers
Take a look at the following 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=" . $example::Y ."n";
?>
It will produce the following output −
public=10 PHP Notice: Undefined property: example::$Y in line 11 private= PHP Fatal error: Uncaught Error: Cannot access private const example::Y