_drupal_discover_files_by_path.php

<?php

/**
 * Recursively scans a directory and its subdirectories, to collect all files
 * that end with the specified extension.
 *
 * The result is keyed by file basename.
 *
 * @param string $dir
 *   E.g. 'sites/all/modules'
 * @param string $extension_with_dot
 *   Extension of files to look for, including the dot.
 *   E.g. '.module'
 *
 * @return string[]
 *   Format: $[$file] = $basename
 *   E.g. $[''modules/system/system.module'] = 'system'
 */
function _drupal_discover_files_by_path($dir, $extension_with_dot) {

  $glob_pattern_suffix = '*{/,' . $extension_with_dot . '}';
  $glob_flags = GLOB_NOSORT | GLOB_BRACE | GLOB_MARK | GLOB_NOESCAPE;
  $extension_strlen_neg = -strlen($extension_with_dot);

  $files = array();
  $dirs_with_slash = array($dir . '/');
  $i = 0;
  while ($i >= 0) {
    // glob() sometimes returns FALSE if no results are found, even
    // if there is no real error.
    foreach (glob($dirs_with_slash[$i--] . $glob_pattern_suffix, $glob_flags) ?: array() as $file_or_subdir_with_slash) {
      if ('/' === $strrchr = strrchr($file_or_subdir_with_slash, '/')) {
        $dirs_with_slash[++$i] = $file_or_subdir_with_slash;
      }
      else {
        $files[$file_or_subdir_with_slash] = substr($strrchr, 1, $extension_strlen_neg);
      }
    }
  }

  return $files;
}

See #2696335 Optimize file_scan_directory() / drupal_system_listing()

Optimized for cases where the number of matches (e.g. every *.module file) is much smaller than the overall number of files.