Allows customers to select membership duration at checkout. Adjusts the amount charged and expiration date of the membership accordingly.

pmpro-select-membership-duration.php

<?php
/**
 * Allow customers to select membership duration at checkout. Adjust the amount charged and expiration date of the membership accordingly.
 *
 * Add this code below to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
 * Requires the Paid Memberships Pro Register Helper Add On to be installed and activated in order to work:
 * https://www.paidmembershipspro.com/add-ons/pmpro-register-helper-add-checkout-and-profile-fields/
 */

/**
 * Instantiate the Register Helper class
 *
 * @return [type] [description]
 */
function my_pmprorh_init() {

    // don't break if Register Helper is not loaded
    if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
        return false;
    }

    // define the fields
    $fields = array();
    $fields[] = new PMProRH_Field(
        'membership_duration',
        'select',
        array(
            'label'     => 'Membership Duration',
            'options'    => array(
                'no' => '1 Year',
                2 => '2 Years',
                3 => '3 Years',
            ),
            'levels' => array( 8 ), // Make sure this is to only show for yearly membership options. Set array number to relevant level ID.
        )
    );
    // add the fields into a new checkout_boxes are of the checkout page
    foreach ( $fields as $field ) {
        pmprorh_add_registration_field(
            'checkout_boxes',
            $field
        );
    }

}
add_action( 'init', 'my_pmprorh_init' );

/**
 * my_pmpro_checkout_level Alters the amount charged and the expiration date of the membership depending on duration selected by user.
 *
 * @param  object $level The $pmpro_level object prior to duration being added.
 *
 * @return object $level The callback function returns the modified $pmpro_level object with the added duration.
 */
function my_pmpro_checkout_level( $level ) {

    $duration = isset( $_REQUEST['membership_duration'] ) ? $_REQUEST['membership_duration'] : '';

    // Bail if no duration is set.
    if ( empty( $duration ) ) {
        return $level;
    }

    // Bail if the user hasn't selected a duration and keep level intact.
    if ( 'no' === $duration ) {
        return $level;
    }

    // Duration for 2 years, give 20% discount.
    if ( '2' === $duration ) {
            $duration = 2;
            $discount_percentage = 0.2;
    }

    // Duration for 3 years, give 25% discount.
    if ( '3' === $duration ) {
        $duration = 3;
        $discount_percentage = 0.25;
    }

    /**
     * Set the duration to selected and adjust the charged amount.
     *
     * Please Note - the new amount will only show upon redirecting to payment gateway page.
     */
    if ( is_int( $duration ) ) {

        // Set the level's duration.
        $level->expiration_number = $duration;
        $level->expiration_period = 'Year';

        // Let's calculate the new price with discount.
        $new_amount = $duration * $level->initial_payment;
        $discount_amount = $new_amount * $discount_percentage;

        // Set the level's new price.
        $level->initial_payment = round( $new_amount - $discount_amount );
    }

    return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level' );

This recipe is included in the blog post on "Add a Membership Duration dropdown to Checkout and Offer a Discount for Extending Membership" at Paid Memberships Pro here: https://www.paidmembershipspro.com/add-a-membership-duration-dropdown-to-checkout-and-offer-a-discount-for-extending-membership/