A possible solution could be a @methodPrefix tag on top of the __get() method, like this:

abstract class ContainerBase {

  private $buffer = array();

  /**
   * @param string $key
   *
   * @return mixed
   *
   * @methodPrefix calc_
   */
  function __get($key) {
    return array_key_exists($key, $this->buffer)
      ? $this->buffer[$key]
      : $this->buffer[$key] = $this->{'calc_' . $key}();
  }
}

Another solution could be to somehow put this into the @return tag.

abstract class ContainerBase {

  private $buffer = array();

  /**
   * @param string $key
   *
   * @return $this->{'calc_' . $key}()
   */
  function __get($key) {
    return array_key_exists($key, $this->buffer)
      ? $this->buffer[$key]
      : $this->buffer[$key] = $this->{'calc_' . $key}();
  }
}

Other ideas?

Or, a smart IDE could do some code evaluation, even without any new doc tags:

• Iterate through the existing property names declared with @property.
• For each property name, evaluate __get($key) as if $key was a constant.
• This way, $function = 'calc_' . $key also becomes a constant.
• $this->$function() or $this->{'calc_' . $key}() can be evaluated like an explicit method call. The IDE can detect if a method calc_xyz() is missing, and complain - except if the class is abstract.
• The type of $v = $this->{'calc_' . $key}() can be determined by looking at the respective calc_*() method.
• Thus, the return type of __get($key) can be determined, separately for each possible value of $key. The IDE can compare this return type to the type declared in the @property tag.