Zend_Loader::loadFile($filename, $dirs=null, $once=false);

/**
 * Loads a PHP file. This is a wrapper for PHP's include() function.
 *
 * $filename must be the complete filename, including any
 * extension such as ".php". Note that a security check is performed that
 * does not permit extended characters in the filename. This method is
 * intended for loading Zend Framework files.
 *
 * If $dirs is a string or an array, it will search the directories
 * in the order supplied, and attempt to load the first matching file.
 *
 * If the file was not found in the $dirs, or if no $dirs were specified,
 * it will attempt to load it from PHP's include_path.
 *
 * If $once is TRUE, it will use include_once() instead of include().
 *
 * @param string    $filename
 * @param string|array $dirs - OPTIONAL either a path or array of paths
 *            to search.
 * @param boolean    $once
 * @return boolean
 * @throws Zend_Exception
 */
public static function loadFile($filename, $dirs = null, $once = false)
{
  self::_securityCheck($filename);
  /**
   * Search in provided directories, as well as include_path
   */
  $incPath = false;
  if (!empty($dirs) && (is_array($dirs) || is_string($dirs))) {
    if (is_array($dirs)) {
      $dirs = implode(PATH_SEPARATOR, $dirs);
    }
    $incPath = get_include_path();
    set_include_path($dirs . PATH_SEPARATOR . $incPath);
  }
  /**
   * Try finding for the plain filename in the include_path.
   */
  if ($once) {
    include_once $filename;
  } else {
    include $filename;
  }
  /**
   * If searching in directories, reset include_path
   */
  if ($incPath) {
    set_include_path($incPath);
  }
  return true;
}

Zend_Loader::loadClass('Container_Tree',
  array(
    '/home/production/mylib',
    '/home/production/myapp'
  )
);

/**
* Loads a class from a PHP file. The filename must be formatted
* as "$class.php".
*
* If $dirs is a string or an array, it will search the directories
* in the order supplied, and attempt to load the first matching file.
*
* If $dirs is null, it will split the class name at underscores to
* generate a path hierarchy (e.g., "Zend_Example_Class" will map
* to "Zend/Example/Class.php").
*
* If the file was not found in the $dirs, or if no $dirs were specified,
* it will attempt to load it from PHP's include_path.
*
* @param string $class   - The full class name of a Zend component.
* @param string|array $dirs - OPTIONAL Either a path or an array of paths
*               to search.
* @return void
* @throws Zend_Exception
*/
public static function loadClass($class, $dirs = null)
{
    if (class_exists($class, false) || interface_exists($class, false)) {
      return;
    }
    if ((null !== $dirs) && !is_string($dirs) && !is_array($dirs)) {
      require_once 'Zend/Exception.php';
      throw new Zend_Exception('Directory argument must be a string or an array');
    }
    // Autodiscover the path from the class name
    // Implementation is PHP namespace-aware, and based on
    // Framework Interop Group reference implementation:
    // http://groups.google.com/group/php-standards/web/psr-0-final-proposal
    $className = ltrim($class, '\\');
    $file   = '';
    $namespace = '';
    if ($lastNsPos = strripos($className, '\\')) {
      $namespace = substr($className, 0, $lastNsPos);
      $className = substr($className, $lastNsPos + 1);
      $file   = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
    }
    $file .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';
    if (!empty($dirs)) {
      // use the autodiscovered path
      $dirPath = dirname($file);
      if (is_string($dirs)) {
        $dirs = explode(PATH_SEPARATOR, $dirs);
      }
      foreach ($dirs as $key => $dir) {
        if ($dir == '.') {
          $dirs[$key] = $dirPath;
        } else {
          $dir = rtrim($dir, '\\/');
          $dirs[$key] = $dir . DIRECTORY_SEPARATOR . $dirPath;
        }
      }
      $file = basename($file);
      self::loadFile($file, $dirs, true);
    } else {
      self::loadFile($file, null, true);
    }
    if (!class_exists($class, false) && !interface_exists($class, false)) {
      require_once 'Zend/Exception.php';
      throw new Zend_Exception("File \"$file\" does not exist or class \"$class\" was not found in the file");
    }
}

if (Zend_Loader::isReadable($filename)) {
  // do something with $filename
}

/**
 * Returns TRUE if the $filename is readable, or FALSE otherwise.
 * This function uses the PHP include_path, where PHP's is_readable()
 * does not.
 *
 * Note from ZF-2900:
 * If you use custom error handler, please check whether return value
 * from error_reporting() is zero or not.
 * At mark of fopen() can not suppress warning if the handler is used.
 *
 * @param string  $filename
 * @return boolean
 */
public static function isReadable($filename)
{
  if (is_readable($filename)) {
    // Return early if the filename is readable without needing the
    // include_path
    return true;
  }
  if (strtoupper(substr(PHP_OS, 0, 3)) == 'WIN'
    && preg_match('/^[a-z]:/i', $filename)
  ) {
    // If on windows, and path provided is clearly an absolute path,
    // return false immediately
    return false;
  }
  foreach (self::explodeIncludePath() as $path) {
    if ($path == '.') {
      if (is_readable($filename)) {
        return true;
      }
      continue;
    }
    $file = $path . '/' . $filename;
    if (is_readable($file)) {
      return true;
    }
  }
  return false;
}

<?php
$loader = new Zend_Loader_PluginLoader();
$loader->addPrefixPath('Zend_View_Helper', 'Zend/View/Helper/')
    ->addPrefixPath('Foo_View_Helper', 'application/modules/foo/views/helpers')
    ->addPrefixPath('Bar_View_Helper', 'application/modules/bar/views/helpers');
?>

<?php
// load 'FormText' helper:
$formTextClass = $loader->load('FormText'); // 'Zend_View_Helper_FormText';
// load 'FormLabel' helper:
$formLabelClass = $loader->load('FormLabel'); // 'Foo_View_Helper_FormLabel'
// load 'FormSubmit' helper:
$formSubmitClass = $loader->load('FormSubmit'); // 'Bar_View_Helper_FormSubmit'
?>

<?php
$loader = new Zend_Loader_PluginLoader(array(
  'Zend_View_Helper' => 'Zend/View/Helper/',
  'Foo_View_Helper' => 'application/modules/foo/views/helpers',
  'Bar_View_Helper' => 'application/modules/bar/views/helpers'
));
?>

<?php
// Store plugins in static registry 'foobar':
$loader = new Zend_Loader_PluginLoader(array(), 'foobar');
?>

<?php
if ($loader->isLoaded('Adapter')) {
  $class  = $loader->getClassName('Adapter');
  $adapter = call_user_func(array($class, 'getInstance'));
}
?>