Using the Date API module (part of the Date module), you can do something like this...

<?php
$form['payment_expirationDate'] = array(
    '#type' => 'date_select',
    '#title' => t('Expiration Date:'),
    '#date_format' => 'm', OR  '#date_format' => 'y',
    '#default_value' => $expirationDate,
    '#date_year_range' => '-1:+10',
    '#required' => TRUE,
    '#date_label_position' => 'within'
 ); 
?>

The date_select type will give you select box form elements for your field. And the #date_format array key allows you to use a PHP date format string to control what select boxes appear and what goes inside them.

Reference

Using the Date API module (part of the Date module), you can do something like this...

<?php
$form['payment_expirationDate'] = array(
    '#type' => 'date_select',
    '#title' => t('Expiration Date:'),
    '#date_format' => 'm', OR  '#date_format' => 'y',
    '#default_value' => $expirationDate,
    '#date_year_range' => '-1:+10',
    '#required' => TRUE,
    '#date_label_position' => 'within'
 ); 
?>

The date_select type will give you select box form elements for your field. And the #date_format array key allows you to use a PHP date format string to control what select boxes appear and what goes inside them.

Reference