class_constants.php

class test {
	const HELLO = 'Hello';
}
class test1 extends test {
	const HELLO = 'Hello, test1';
}
class test2 extends test {
	const HELLO = 'Hello, test2';
}

$test  = new test();
$test1 = new test1();
$test2 = new test2();

echo $test::HELLO . "\r\n";
echo $test1::HELLO . "\r\n";
echo $test2::HELLO . "\r\n";

/**
 * RESULT
 * ------------
 * Hello
 * Hello, test1
 * Hello, test2
 */