Creates a log at /paid-memberships-pro/logs/level-change.txt for whenever a membership level is changed.

my_pmpro_after_change_membership_level_log.php

<?php

// Copy from below here...

/*
 * Creates a log at /paid-memberships-pro/logs/level-change.txt for whenever a membership
 * level is changed.
 */
function my_pmpro_after_change_membership_level_log( $level_id, $user_id, $cancel_level ) {
	$logstr = "Logged On: " . date_i18n("m/d/Y H:i:s") . "\n";
	$logstr .= "Level: " . $level_id . "\n";
	$logstr .= "User ID: " . $user_id . "\n";
	$logstr .= "Cancel Level: " . $cancel_level . "\n";
	$logstr .= "Request URI: " . $_SERVER['REQUEST_URI'] . "\n";
	$logstr .= "\n-------------\n";
	
	$loghandle = fopen(PMPRO_DIR . "/logs/level-change.txt", "a+");
	fwrite($loghandle, $logstr);
	fclose($loghandle);
}
add_action( 'pmpro_after_change_membership_level', 'my_pmpro_after_change_membership_level_log', 10, 3 );