prorated_memberships.module

<?php
/**
 * @file
 * Prorates membership fees in CiviCRM for fixed term memberships
 */

/**
 * Implements hook_civicrm_membershipTypeValues()
 * 
 * Will prorate the membership on a monthly step basis. It will also add an
 * additional year's membership based on the rollover date that is set per membership type.
 */

function prorated_memberships_civicrm_membershipTypeValues( &$form, &$membershipTypeValues ) {  
  // Ignore the thank you page.
  if ($form->getVar('_name') == 'ThankYou') {
    return;
  }
  
  if (!in_array(get_class($form), array(
            'CRM_Contribute_Form_Contribution',
            'CRM_Contribute_Form_Contribution_Main',
          ))) {
    return;
  }
    
  foreach ( $membershipTypeValues as &$values) {
    $results = civicrm_api("MembershipType","get", array('version'=>'3','sequential'=>'1','id'=>$values[id]));
      
    // Only fixed term memberships
    if(($results['is_error'] > 0 ) || $results['values'][0]['period_type'] != 'fixed') { return; }
    
    // Starting date as set in Member type
    $start_month = substr($results['values'][0]['fixed_period_start_day'], 0, strlen($results['values'][0]['fixed_period_start_day']) - 2);
    
    // Rollover date
    $rollover_month = substr($results['values'][0]['fixed_period_rollover_day'], 0, strlen($results['values'][0]['fixed_period_rollover_day']) - 2);
    
    $today = getdate();
    
    // Calcuate the number of months remaining in the membership period
    $months = $start_month - $today['mon'];
    if ($months < 0) { $months = 12 + $months; }
    $ratio = $months/12;
    
    if (($today['mon'] >= $rollover_month) && ($today['mon'] <= $start_month)) {
      $amount = round($values['minimum_fee'] + $values['minimum_fee'] * $ratio, 2);
    } else {
      $amount = round($values['minimum_fee'] * $ratio, 2);
    }
    
    $values['minimum_fee'] = $amount;
  }  
}