gistfile1.php

<?php

abstract class write_once_class 
{	
	function __get($key) {
		if(property_exists(get_class($this), $key)) {
			return $this->$key;
		} else {
			throw new Exception("Can't get a variable which isn't declared.");
		}
	}
	
	function __set($key, $value) {
		if(property_exists(get_class($this), $key) && is_null($this->$key)) {
			$this->$key = $value;
		} else {
			throw new Exception("Can't set a variable which has been previously set or isn't declared.");
		}
	}
}

class my_class extends write_once_class
{
    protected $foo = null;
    protected $bar = null;
    protected $baz = null;
}

$c = new my_class();
$c->foo = 33;
print $c->foo;
$c->foo = 39;