let's go ahead and be thorough and explicit

	if ( ! is_php(5))

becomes

	if ( ! is_php('5.0.0'))

ref: http://codeigniter.com/bug_tracker/bug/8475/
diff --git a/system/codeigniter/CodeIgniter.php b/system/codeigniter/CodeIgniter.php
index 1671287..7b945d2 100644
--- a/system/codeigniter/CodeIgniter.php
+++ b/system/codeigniter/CodeIgniter.php
@@ -134,7 +134,7 @@
  *  Note: The Loader class needs to be included first
  *
  */
-if ( ! is_php(5))
+if ( ! is_php('5.0.0'))
 {
 	load_class('Loader', FALSE);
 	require(BASEPATH.'codeigniter/Base4'.EXT);