static_wtf.php

<?php
class Foo {
  protected static $blah;
  static function stuff() {
    static $cuux = null;
    if ($cuux === null) {
      echo "It's null.\n";
      $cuux = 42;
    } else {
      echo "It's not null anymore.\n";
    }
  }
  static function stiff() {
    if (self::$blah === null) {
      echo "It's null.\n";
      self::$blah = 42;
    } else {
      echo "It's not null anymore.\n";
    }
  }
}

class Bar extends Foo {
}

Foo::stuff(); // It's null.
Foo::stuff(); // It's not null anymore.
Bar::stuff(); // It's null.
Bar::stuff(); // It's not null anymore.
echo "---\n";
Foo::stiff(); // It's null.
Foo::stiff(); // It's not null anymore.
Bar::stiff(); // It's not null anymore.
Bar::stiff(); // It's not null anymore.