$customField['option1'] = 'value1';
$customField['option2'] = 'value2';
$customField['option3'] = 'value3';

echo generateSelect('Category', $customField)

function generateSelect($name = '', $options = array()) {
    $html = '<select name="'.$name.'">';
    foreach ($options as $option => $value) {
        $html .= '<option value='.$value.'>'.$option.'</option>';
    }
    $html .= '</select>';
    return $html;
}