Lazy Strings class

lazystring.php

<?php

   // accepts default string and test function.
   // test function is tweaked everytime object of class LazyStrings is casted to string, 
   // if test function returns FALSE/NULL default string is returned, otherwise the result of 
   // test.

class LazyString {

   private $_default, $_tweak;
   
   public function __construct($default_string, $tweak) {
      list($this->_default, $this->_tweak) = array($default_string, $tweak);
   }

   public function __toString() {
      if ($this->_tweak instanceof Closure) {
         $result = call_user_func($this->_tweak);
         return ($result!==NULL && $result!==FALSE) ? $result : $this->_default;
      }
      // TODO: accept callbacks in strings ('checkMyVariable') and arrays of
      // two types: array($someObj, 'method') and array('someClass', 'someStaticMethod')
      return $_default;
   }
}

This is extremely useful in the following case:

<?php

// some Closure which generates HTML from MD
$myDescriptionFieldPreprocessor = Options::get('someModel.description.preprocessor'); 

$myMarkdownDescription = new LazyString($someModel->fetchOne(/*...*/)->description, $myDescriptionFieldProcessor);

// And.. forget about the routine of generating HTML from Markdown! 
// It will be done implicitly (while rendering the template, as an example)
?>