Message: implode(): Argument #2 ($array) must be of type ?array, string given

$query_span = $this->span_centralregion->get();
        foreach($query_span->result() as $spn) {
            $span[$spn->span_id] = $spn->span_name;   
        } 
 
$data = array(
            'page' => 'add',
            'row' => $ticket,
            'span' => $span, 'selectedspan' => null,
        );

public function add($post) { 
 
    $params = [ 
 
    'span_id' => implode("<>", $post['span']), 
 
    ]; 
}

<div class="form-group">
    <label><strong>Span ID*</strong></label>
    <?php echo form_dropdown('span', $span, $selectedspan,
    ['class' => 'form-control', 'required' => 'required', 'id' => 'select2-demo-3', 'data-plugin' => 'select2', 'multiple' => 'multiple', 'data-options' => '{ maximumSelectionLength: 2 }']) ?>
</div> 

// mungkin deklarasiin dulu $span sebagai array kosong, karena kemungkinan $query_span kosong
$span = [];
$query_span = $this->span_centralregion->get();
        foreach($query_span->result() as $spn) {
            $span[$spn->span_id] = $spn->span_name;   
        } 

$data = array(
            'page' => 'add',
            'row' => $ticket,
            'span' => $span ?? [], 
            'selectedspan' => null,
        );

// untuk jaga jaga
public function add($post) { 
    if(!is_array($post['span'])) return 'bukan array';
    $params = [ 
 
    'span_id' => implode("<>", $post['span']), 
 
    ]; 
}