使用PHP处理分叉

I have copied the below code and on running the code it always shows

parent

as output.But the source file says it will display output as

child

parent

.Please advice me "How to implement threading in PHP".

<?php
 if (! function_exists('pcntl_fork')) die('PCNTL functions not available on this PHP installation');
 $pid = pcntl_fork();
 if($pid) {
   echo "parent
";
 }
 else {
   echo "child
";
 }
?>

Note:While running in cli getting an error

PHP Warning: Module 'pcntl' already loaded in Unknown on line 0

Source:http://www.electrictoolbox.com/article/php/process-forking/