Multi tool use

create a multiselect dropdown with 3-story levels

I am making a multi-story multiselect dropdown with 3 level, level 1 and 2 is successfully performed, but for level 3 error (error array to string).How to solve it?
I attach the following code:

model :

function getZonaTree2() {
$types = $this->getZonaType();
$tree = array(''=>'-- Region --');
$this->getListTreeParent($listZona, 0, 0);
foreach($listZona as $r) {
if($r->LEVEL==1) {
$tree[$r->ZONA_NAME] = array();
$zona_parent_1 = $r->ZONA_NAME;
}
if($r->LEVEL==2) {
$tree[$zona_parent_1][$r->ZONA_NAME] = array();
$zona_parent_2 = $r->ZONA_NAME;
}
if($r->LEVEL==3) {
$tree[$zona_parent_1][$zona_parent_2][$r->ZONA_ID] = $r->ZONA_NAME." (".$r->ZONA_CAPITAL.") - ".$types[$r->TYPE];
}
}
return $tree;
}


view :

<?php
echo form_multiselect('zona_id', $this->Zona_model->getZonaTree2(), array(), "size=8 id='zona_id'");
?>


display error :

the display should :

$this->Zona_model->getZonaTree2()

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