cannot unserialize the string into form_value

i’m trying to print on screen tha data that are into the columns ‘form_value’, unserialize() php method doesn’t work inside the while cycle. i tried the method with a simple program: <?php $a='(content_of_form_value_column)’; var_dump(unserialize($a)); ?> and like this work correctly. if i try to do the same with all the record of the table the result is bool(false). i tried with a pattern and preg_match_all() method but some form field are textarea (description of an event) and the pattern doesn’t work all the time because the user can write everything.
On internet i found only other person using the unserialized() method.