Zend Multiselect Element is posting only one selected values.

Question

I am creating multiple select element like this and it is showed successfully on form:

$element = new Zend_Form_Element_Multiselect('clinics');
$element->setLabel("Clinics");
$element->setAttrib( 'style','width: 240px' );
$element->setMultiOptions( array( '1'=>'clinic1', '2'=>'clinic2' ) );

After rendering above element it shows the following html in html source:

<select name="clinics[]" id="clinics" multiple="multiple" style="width: 240px" size="5" class="required" tabindex="41">
    <option value="1" label="clinic1">clinic1</option>
    <option value="2" label="clinic2">clinic2</option>
</select>

But when I submit the form with two selected fields and print_r the result like this:

    $request = $this->getRequest();
    $form = new Patient_Form_Patient( $formOptions );

    if ( $request->isPost() ) {
        if ( $form->isValid( $request->getPost() ) ) {
            $values = $form->getValues();
            print_r($values);die();
        }
    } 

It stores only first selected option in array but not all selected elements:

Array
( 
    [clinics] => Array
        (
            [0] => 1
        )

    [save] => Submit
)

Can someone help that how can I submit multiple values ?