No proration for the initial payment & set the start date for the recurring payments to the 1st of the month

pmpro-start-of-month-payments.php

<?php
/**
 * Plugin Name: UNprorated initial membership payment
 * Plugin URI: http://www.paidmembershipspro.com/wp/pmpro-customizations/
 * Description: Will prorate the membership payment from today until the 1st of the next month.
 * Version: .2.1
 * Author: Thomas Sjolshagen @ Stranger Studios <thomas@eighty20results.com>
 */
/**
 * Keep the membership level payment same as a full month.
 *
 * @param stdClass $level - The membership level definition to update
 *
 * @return stdClass (level)
 */
function e20r_prorate_initial_payment( $level ) {
	if ( false == pmpro_hasMembershipLevel() ) {
		// user does not have level already, but we want to prorate initial payment for rest of the month
		// today
		$today = current_time( 'timestamp' );

		// when would the first subscription payment be
		$next_payment_date = strtotime( 'first day of next month', current_time( 'timestamp' ) );

		// how many days left
		// $days_left = ceil( ( $next_payment_date - $today ) / DAY_IN_SECONDS );
		// convert to percent
		// $percent = $days_left / ( $next_payment_date - strtotime( 'first day of this month', current_time( 'timestamp' ) ) );
		// Let's remove the proration
		$percent = $days_left / $days_left;
		$level->initial_payment = round( $level->initial_payment * $percent, 2 );
	}
	return $level;
}
add_filter( 'pmpro_checkout_level', 'e20r_prorate_initial_payment', 11, 1 );

/**
 * Set the startdate for the next payment for use by the payment gateway (i.e. 1st of next month)
 *
 * @param   string      $startdate      The default start date for the subscription plan
 * @param   MemberOrder $order          The order object
 *
 * @return string               The date to use
 */
function e20r_prorate_set_startdate( $startdate, $order ) {
	if ( false == pmpro_hasMembershipLevel() ) {
		$startdate = date( 'Y-m-01', strtotime( date( 'Y-m-01', current_time( 'timestamp' ) ) . ' + ' . $order->BillingFrequency . ' ' . $order->BillingPeriod ) ) . 'T0:0:0';
	}
	return $startdate;
}
add_filter( 'pmpro_profile_start_date', 'e20r_prorate_set_startdate', 11, 2 );