foobar.php

<?php
class Foobar {
        static $global_value;

        private $_local_value;

        public function __construct($local_val = null) {
                if(false == is_null($local_val)) {
                        $this->_local_value = $local_val;
                }
        }

        public function getValue() {
                return isset($this->_local_value) ? $this->_local_value : Foobar::$global_value;
        }
}

Foobar::$global_value = 'global_value';
echo Foobar::$global_value . "\n"; //global_value 
$F = new Foobar();
echo $F->getValue() . "\n"; //global_value
$F = new Foobar('new value');
echo $F->getvalue() . "\n"; //new value
?>