Description:
------------
Using file_exists fails to find valid files within include_path when using an __autoload. Removing file_exists() the required file is found and loaded.

Safe mode is not enabled.

Reproduce code:
---------------
<?php
function __autoload( $classname )
{
	$file = str_replace( '_', DIRECTORY_SEPARATOR, $classname ).'.php';

	if( file_exists( $file ) )
	{
		require( $file );
		return true;
	}
	else
	{
		return FALSE;
	}
}
# Sets include path to the current directory just for easy testing.
set_include_path( realpath( dirname(__FILE__) ).DIRECTORY_SEPARATOR.PATH_SEPARATOR.get_include_path() );

$test = new Prefix_ClassName();
?>

Expected result:
----------------
File included class initialized.

Actual result:
--------------
Fatal error: Class 'Prefix_ClassName' not found.