desirelabs on "Impossible to delete a user meta"

I'm a beginner with WordPress plugin api, php and ajax.
I'm coding a training plugin for myself to save posts article to read them later, like favourites on web browser, with folders.

I use oop to do so.

I assumed to create a new user meta called 'favoris', wich is an array with the users created folders.

I got something like that for getting favs, creating folders and deleting'em.

/**
 * Get favorites
 */
public function fp_ajax_getFav(){
	$current_user = wp_get_current_user();
	$userid = $current_user->ID;
	$folder = get_user_meta($userid, 'favoris');
	$content =  '<div id="pfolders">';
	$content .= '<h3 class="widget-title">Favoris</h3>';
	$content .= '<form id="pform" action="#" method="POST">';
	$content .= '<input type="text" placeholder="nouveau dossier" class="foldername">';
	$content .= '<input type="submit" class="addfolder" value="+">';
	$content .= '</form>';

	if (!empty($folder)) {
		$content .= '<a href="#" class="clear_all" style="display:inline">Tout supprimer</a>';
		$content .= '<span class="empty" style="display:none"><i>Aucun favoris.</i></span>';
	}
	else{
		$content .= '<a href="#" class="clear_all" style="display:none">Tout supprimer</a>';
		$content .= '<span class="empty" style="display:inline"><i>Aucun favoris.</i></span>';
	}

	$content .= '<ul id="accordion" class="favoris">';

	if (!empty($folder)) {
		$msg = '';
		foreach ($folder as $key) {
			foreach ($key as $k => $value) {
				$content .= '<li class="dossier dossier-' . $k . '"><span><a href="#" class="erase" data-key="' . $k . '"><i class="fa fa-times" style=""></i></a> <a href="#" class="dossier-nom-' . $k . '">' . ucwords($value['titre']) . '</a></span></li>';
			}
		}
	}
	else{
	}
	$content .= '</ul>';
	$content .= '</div>';
	return $content;
}

/**
 * Add folder
 */
public function fp_ajax_addFolder(){
	$current_user = wp_get_current_user();
	$userid = $current_user->ID;
	$meta = get_user_meta($userid, 'favoris');
	if (empty($meta)) {
		if ($_REQUEST['foldername'] !== '') {
			$folder[0]['titre'] = $_REQUEST['foldername'];
			add_user_meta($userid, 'favoris', $folder);
			var_dump($folder);
			echo ucwords($_REQUEST['foldername']);
			die();
		}
	}
	if (!empty($meta)) {
		$folder = $meta[0];
		if ($_REQUEST['foldername'] != '') {
			foreach ($folder as $key => $value) {
				$i = count($folder);
				if ($value['titre'] != $_REQUEST['foldername']) {
					$folder[$i]['titre'] = $_REQUEST['foldername'];
					update_user_meta($userid, 'favoris', $folder);
					var_dump($folder);
					echo ucwords($_REQUEST['foldername']);
					die();
				}
			}
		}
	}
}

/**
 * Remove folder
 */
public function fp_ajax_removeFolder(){
	if ( ( !isset($_POST['folderId']) || empty($_POST['folderId']) ) && ( !isset($_POST['folderName']) || empty($_POST['folderName']) ) ) {
		echo 'Pas de dossier spécifié';
		die();
	}
	else{
		$id = $_POST['folderId'];
		$name = $_POST['folderName'];
		$current_user = wp_get_current_user();
		$userid = $current_user->ID;
		$folder = get_user_meta($userid, 'favoris');
		$folderToErase = $folder[0][$id];
		try {
			delete_user_meta($userid, 'favoris', $folder[0][$id]);
			echo $folderToErase['titre'];
			//print_r($folder[0][$id]['titre']);
		}
		catch (Exception $e) {
			echo 'not ok';
			var_dump($folder);
			return $e->getMessage();
		}
		die();
	}
}

As far as I get or create, everything goes fine. But when I want to remove a folder by key ($id), it doesn't. But when I echo the $folderToErase, the id's right.

Maybe some help ?

Just in case, I also opened a ticket on stackexchange :
http://wordpress.stackexchange.com/questions/136499/how-to-delete-user-meta-by-key