Simple registry. Static class so it can be autoloaded and accessed very easily.

Registry.php

<?php

/**
 * Registry
 *
 * Simple class for accessing shared values
 */
class Registry
{
    /**
     * Holds registry values
     *
     * @var array
     */
    protected static $values = array();

    /**
     * Holds readonly keys
     *
     * @var array
     */
    protected static $readonly = array();

    /**
     * Sets a value into registry
     *
     * @param string $key
     * @param mixed $value
     * @param bool $readonly
     *
     * @return bool
     */
    public static function set($key, $value, $readonly = true)
    {
        if (in_array($key, self::$readonly)) {
            return false;
        }

        if ($readonly === true) {
            self::$readonly[] = $key;
        }

        self::$values[$key] = $value;

        return true;
    }

    /**
     * Returns a value from the registry
     *
     * @param string $key
     */
    public static function get($key)
    {
        return (array_key_exists($key, self::$values)) ? self::$values[$key] : null;
    }
}