<?php
class Foo
{
	const BAR = 'Hallo';
	const BAZ = 'Welt!';
		
	/**
	 * 
	 * Returns the value of a class constant
	 * @param string $const Name of the constant
	 * @param string $class [optional] Name of the class (if not set, the constant will be searched in this class) 
	 * @return mixed|false The value of the constant or false if it does not exists
	 */
	public static function get_constant( $const = '', $class_name = __CLASS__ ){
	
		if( empty( $const ) || ! is_string( $const ) || ! class_exists( $class_name ) )
			return FALSE;
			
		$reflection = new ReflectionClass( $class_name );
		
		return $reflection->getConstant( $const );
		
	}
	
}

class Bar
{
	const FOO = 'Horst';
}

$consts = array( 'BAR', 'BAZ', 'FOO' );
foreach( $consts as $const )
	var_dump( Foo::get_constant( $const ) );
	
var_dump( Foo::get_constant( 'FOO', 'Bar' ) );

var_dump( Foo::get_constant( 'TEST', 'Baz' ) );