[PHP] static not working as expected

[Purushotham Komaravolu]

<?php

    class Counter {
        var $counter = 0;
        
        function increment_and_print()
        {
                print ++$this->counter;
                print "\n";
        }    
    }
    
    
    class SingletonCounter {
        static $m_instance = NULL;  // throwing error here
        
        function Instance()
        {
                if (self::$m_instance == NULL) {
                    self::$m_instance = new Counter();
                }
                return self::$m_instance;
        }    
    }
    
    SingletonCounter::Instance()->increment_and_print();
    SingletonCounter::Instance()->increment_and_print();
    SingletonCounter::Instance()->increment_and_print();
    
?>


is throwing the following error
Parse error: parse error, expecting `T_OLD_FUNCTION' or `T_FUNCTION' or `T_VAR' or 
`'}'' in /singleton.php on line 15