Stop members downgrading their membership level

checkout/stop-members-from-downgrading-level.php

@@ -0,0 +1,63 @@
+<?php
+/**
+ * Define a level hierarchy and stop members from downgrading their membership level
+ *
+ * This recipe is not compatible with sites that allow multiple memberships per user
+ *
+ * title: Stop members downgrading their membership level
+ * layout: snippet
+ * collection: checkout
+ * category: registration-check
+ *
+ * You can add this recipe to your site by creating a custom plugin
+ * or using the Code Snippets plugin available for free in the WordPress repository.
+ * Read this companion article for step-by-step directions on either method.
+ * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
+ */
+
+function my_pmpro_tiered_levels_prevent_downgrade( $okay ) {
+	// Bail early if something else isn't okay.
+	if ( ! $okay ) {
+		return $okay;
+	}
+
+	// get user's level
+	$member_level = pmpro_getMembershipLevelForUser();
+
+	// If the user doesn't have a membership level carry on with checkout.
+	if ( false === $member_level ) {
+		return $okay;
+	}
+
+	// member's current level ID
+	$member_level_id = $member_level->id;
+
+	// Get level for checkout.
+	$checkout_level = pmpro_getLevelAtCheckout();
+	$checkout_level_id = $checkout_level->id;
+
+	// bail if renewal 
+	if( $checkout_level_id === $member_level_id ) {
+		return $okay;
+	}
+
+	// Specify the level hierarchy, lowest to highest
+	$level_order = array( 1, 2, 3, 4, 5 );
+
+	// figure out where the levels rank, bail if not in level order
+	$checkout_level_rank = array_search( $checkout_level_id, $level_order );
+	$member_level_rank = array_search( $member_level_id, $level_order );
+	if ( false === $checkout_level_rank || false === $member_level_rank ) {
+		return $okay;
+	}
+
+	// are they checking out for a lower level?
+	if( $member_level_rank > $checkout_level_rank ) {
+		$okay = false;
+		// show an error message on checkout page
+		pmpro_setMessage( 'You are not allowed to checkout for this membership level.', 'pmpro_error' );
+	}
+
+	return $okay;
+}
+add_filter( 'pmpro_registration_checks', 'my_pmpro_tiered_levels_prevent_downgrade' );