sottise

YEAY ! Finally, it works !!

I used the code mentioned here .

<?php
if ( ! function_exists( 'post_is_in_descendant_category' ) ) {
	function post_is_in_descendant_category( $cats, $_post = null ) {
		foreach ( (array) $cats as $cat ) {
			// get_term_children() accepts integer ID only
			$descendants = get_term_children( (int) $cat, 'category' );
			if ( $descendants && in_category( $descendants, $_post ) )
				return true;
		}
		return false;
	}
}
?>
<h1 <?php if ( in_category( 'fruits' ) || post_is_in_descendant_category( ID ) ) {echo 'class="fruits"';}
	  if ( in_category( 'vegetables' ) || post_is_in_descendant_category( ID ) ) {echo 'class="vegetables"';}
	  	?>><?php the_category(' / '); ?></h1>

Happy me ??
Anyway, thank you so much for your help cykeltomte, I really appreciated it ! ??

well ! The fatal error is gone, but… It doesn’t recognize the class…
I am trying to figure it out by searching here

still works for the first category fruits but not for the other one (vegetables)… Argh…

I’ve added the ; missing, but still have the same fatal error :/

<?php $q_cat = get_query_var('cat');
$cat = get_category( $q_cat );
$parent_cat = $cat->category_parent; ?>
	     <h1 <?php
if ( in_category( 'fruits' ) || post_is_in_descendant_category( $parent_cat ) ) {echo 'class="fruits"';}
if ( in_category( 'vegetables' ) || post_is_in_descendant_category( $parent_cat ) ) {echo 'class="vegetables"';}
?>><?php the_category( ' / ' ); ?></h1>

The weird things, it’s that if I use only one if ( in_category, it works fine.

Like :

<?php $q_cat = get_query_var('cat');
$cat = get_category( $q_cat );
$parent_cat = $cat->category_parent; ?>
	     <h1 <?php
if ( in_category( 'fruits' ) || post_is_in_descendant_category( $parent_cat ) ) {echo 'class="fruits"';}
?>><?php the_category( ' / ' ); ?></h1>

Here is the code I’ve tried, but still have a fatal error :/

<?php $q_cat = get_query_var('cat');
	$cat = get_category( $q_cat );
	$parent_cat = $cat->category_parent; ?>
	                <h1 <?php if ( in_category( 'fruits' ) || post_is_in_descendant_category( $parent_cat ) ) {echo 'class="fruits"';}
if (in_category( 'vegetables' ) || post_is_in_descendant_category( $parent_cat ) ) {echo 'class="vegetables"';}
?>><?php the_category( ' / ' ) ?></h1>

oh, wait, I try the new one

Yeay ! Thanks ! But I still have a problem…

It works fine when I use it alone!
But have a fatal error when I do this :

<?php $cat = get_query_var('cat'); ?>
	                <h1 <?php
if ( in_category( 'fruits' ) || post_is_in_descendant_category( $cat ) ) {echo 'class="fruits"';}
if ( in_category( 'vegetables' ) || post_is_in_descendant_category( $cat ) ) {echo 'class="vegetables"';}
 ?>><?php the_category( ' / ' ) ?></h1>

The ID of the parent category actually.

Where do I have to put your code ?

Thanks cykeltomte !
But yep, indeed, I have an error message with this code.

Anyway, I’ve tried a new one and it’s works fine.
I used in_category instead of is_category.

if ( in_category( 'fruits' ) || post_is_in_descendant_category( 6 ) ) {echo 'class="fruits"';}

Now, I am searching how to replace the category ID, to automatically add the subcategory without entering the category ID. ??