Form helper doesn't load

Hello everbody, i am new to the forum but worked with codegniter a lot of times in the past few years.

I have a strange issue, when i am calling the function form_open() from the form helper, i got an php error:

Fatal error: Call to undefined function form_open() in /public_html/application/views/home/homepage_test.php on line 91
A PHP Error was encountered

Severity: Error

Message: Call to undefined function form_open()

Filename: home/homepage_test.php

Line Number: 91

Backtrace:

I call the function form in the autoloader, and any other model, libraries of helpers are loaded fine. So i have tried to remove the form loader from the autolader and load them manually in the controller before i am calling the view, but this also doesn't work. Do someone from the forum maybe have an idea what the error is?

I am using PHP 5.5.25