Thank you,
I'd already contructed the php code of that input field. That wasn't the problem here:
$antwoordAC0404a = value('AC04_04a', 'text');
$antwoordAC0404a = trim($antwoordAC0404a);
replace('%antwoordAC0404a%', $antwoordAC0404a);
And %antwoordAC0404a% in the matrix question.
The problem is that I can't get the selected items of the multiple choice question in the matrix question. So if not selected then it must not displayed in the matrix question.