It can be implemented for example as follow:
<?php
$dir = '/path/to/some/dir/'; // notice: trailing slash!
if ($handle = opendir($dir)) {
while (false !== ($entry = readdir($handle))) {
if (is_dir($dir . $entry) && strtolower(substr($entry, 0, 2)) == 'a_') {
rmdir($dir . $entry);
}
}
closedir($handle);
}
?>