phpÖÐµÄ Àà³£Á¿ Class Constants

It is possible to define constant values on a per-class basis remaining the same and unchangeable.
Constants differ from normal variables in that you don't use the $ symbol to declare or use them. Like static members, constant values cannot be accessed from an instance of the object (using $object::constant).
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Àý×Ó 19-15. Defining and using a constant
<?php
class MyClass
{
    const constant = 'constant value';
    function showConstant() {
        echo  self::constant . "\n";
    }
}
echo MyClass::constant . "\n";
$class = new MyClass();
$class->showConstant();
// echo $class::constant;  is not allowed
?>