Value.php

<?php

/**
 * Allows value objects to expose their properties while maintaining immutability.
 * 
 * Inject your properties in your constructor.
 * Define your properties in the docblock for your class with {@code @property}.
 * Mutability is just a bug waiting to happen.
 */
abstract class Value
{
    /**#@+
     * Error messages
     * 
     * @var string
     */
    private static $PRIVATE_PROPERTY   = "Cannot access private property";
    private static $UNDEFINED_PROPERTY = "Undefined property:";
    /**#@- */
    
    /**
     * Magic getter
     * 
     * @param string $aProperty
     */
    final public function __get($aProperty)
    {
        if (property_exists($this, $aProperty)) {
            return $this->{$aProperty};
        }
        $this->triggerError(static::$UNDEFINED_PROPERTY, $aProperty);
    }
    
    /**
     * Magic setter
     * 
     * @param string $aProperty
     * @param mixed  $aValue
     */
    final public function __set($aProperty, $aValue)
    {
        if (property_exists($this, $aProperty)) {
            $this->triggerError(static::$PRIVATE_PROPERTY, $aProperty);
        }
        $this->triggerError(static::$UNDEFINED_PROPERTY, $aProperty);
    }
    
    /**
     * Trigger an {@code ErrorException}
     * 
     * @param  string $aMessage
     * @param  string $aProperty
     * @throws ErrorException
     */
    private function triggerError($aMessage, $aProperty)
    {
        $backtrace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS);
        $aMessage  = sprintf($aMessage . " %s::$%s", __CLASS__, $aProperty);
        throw new ErrorException($aMessage, 0, E_USER_NOTICE, $backtrace[1]['file'], $backtrace[1]['line']);
    }
}