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PHP Class Constants
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 . "
"; 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 . "
"; 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. "
"; echo "private=" . $s1->Y ."
"; echo "private=" . $example::Y ."
"; ?>
Output
This will produce following result. −
1public=10 PHP Notice: Undefined property: example::$Y in line 11 private=PHP Fatal error: Uncaught Error: Cannot access private const example::Y