problem with single quote in input field

It's the combination of form_input and set_value. Both functions "escape" the value.
You can turn off html escaping in set_value, by adding a third parameter:

PHP Code:

set_value('mo',$projets['mo'],FALSE) 


See if that does the trick.
Otherwise, build the input field with plain html. Then it's possible to use set_value for repopulating the field after form validation.

PHP Code:

<input type="text" name="test" value="<?= set_value('test','default_value');?>" size=64 maxlength=255 />