Create sequential membership numbers. Display on user account page, edit profile page,memberslist page and membership card and include in memberslist csv export.

sequential-numeric-member-number.php

<?php

/**
 * Sequential Member Numbers
 * Member numbers are automatically increased from the value set for $starting_member_number after each member registration
 *
 * Add this code to your PMPro Customizations Plugin
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 * 
 * Requirements:
 * Register Helper - https://www.paidmembershipspro.com/add-ons/pmpro-register-helper-add-checkout-and-profile-fields/
 * Membership Card Add On - https://www.paidmembershipspro.com/add-ons/pmpro-membership-card/
 * Custom Membership Card template - https://gist.github.com/ipokkel/c3a93a5718aa5646b80adff814ed76d4
*/

/**
 * Generate member_number when a user is registered.
 *
 * Existing members already in the system will not receive a membership number
 * This function can be, and is, called elsewhere to generate a membership number
 * for existing members that doesn't have a members number
*/
function generate_member_number( $user_id ) {
	$starting_member_number = 100000; // set starting number here

	$current_member_number = get_option( 'pmpro_seq_current_member_num' );

	if ( empty( $current_member_number ) ) {
		$current_member_number = $starting_member_number;
	}

	update_user_meta( $user_id, 'member_number', $current_member_number );
	update_option( 'pmpro_seq_current_member_num', $current_member_number + 1 );
	
	return $current_member_number;
}
add_action( 'user_register', 'generate_member_number' );

//Show it on the membership account page.
add_action( 'pmpro_account_bullets_bottom', 'pmpromn_pmpro_account_bullets_bottom' );
add_action( 'pmpro_invoice_bullets_bottom', 'pmpromn_pmpro_account_bullets_bottom' );
function pmpromn_pmpro_account_bullets_bottom() {
	if ( is_user_logged_in() ) {
		global $current_user;

		//get member number
		$member_number = get_user_meta( $current_user->ID, 'member_number', true );

		// create on the fly
		if ( empty( $member_number ) ) {
			// lets generate it on the fly.
			$member_number = generate_member_number( $current_user->ID );
			// fetch fido
			$member_number = get_user_meta( $current_user->ID, 'member_number', true );
		}

		//show it
		if ( ! empty( $member_number ) ) {
			?>
				<li><strong><?php _e( 'Member Number', 'paid-memberships-pro' ); ?>:</strong> <?php echo $member_number; ?></li>
			<?php
		}
	}
}



//show member_number in confirmation emails
add_filter( 'pmpro_email_filter', 'pmpromn_pmpro_email_filter', 30, 2 );
function pmpromn_pmpro_email_filter( $email ) {
	global $wpdb;

	//only update admin confirmation emails
	if ( strpos( $email->template, 'checkout' ) !== false ) {
		if ( ! empty( $email->data ) && ! empty( $email->data['user_login'] ) ) {
			$user = get_user_by( 'login', $email->data['user_login'] );
			if ( ! empty( $user ) && ! empty( $user->ID ) ) {
				$member_number = get_user_meta( $user->ID, 'member_number', true );

				if ( ! empty( $member_number ) ) {
					$email->body = str_replace( '<p>Membership Level', '<p>Member Number:' . $member_number . '</p><p>Membership Level', $email->body );
				}
			}
		}
	}

	return $email;
}


//Show member number in Members List and Members List  CSV export
add_filter( 'pmpro_members_list_csv_extra_columns', 'my_pmpro_members_list_csv_extra_columns', 10 );
function my_pmpro_members_list_csv_extra_columns( $columns ) {
	$columns['member_number'] = 'my_extra_column_member_number';

	return $columns;
}

function my_extra_column_member_number( $user ) {
	if ( ! empty( $user->metavalues->member_number ) ) {
		return $user->metavalues->member_number;
	} else {
		generate_member_number( $user->ID );
		return $user->metavalues->member_number;
	}
}

//Show member number in Members List
add_action( 'pmpro_memberslist_extra_cols_header', 'my_pmpro_memberslist_extra_cols_header' );
function my_pmpro_memberslist_extra_cols_header() {
	?>
	<th><?php _e( 'Member Number', 'paid-memberships-pro' ); ?></th>
	<?php
}

//columns
add_action( 'pmpro_memberslist_extra_cols_body', 'my_pmpro_memberslist_extra_cols_body' );
function my_pmpro_memberslist_extra_cols_body( $theuser ) {
	if ( empty( $theuser->member_number ) ) {
		// lets generate it on the fly.
		generate_member_number( $theuser->ID );
	}
	?>
	<td>
	<?php
	if ( ! empty( $theuser->member_number ) ) {
		echo $theuser->member_number;
	}
	?>
	</td>
	<?php
}

// Create register helper field to show member number on profile edit screen for admins
add_action( 'init', 'my_pmprorh_init_member_number' );
function my_pmprorh_init_member_number() {
	//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(
		'member_number', // input name, will also be used as meta key
		'text',      // type of field
		array(
			'label'   => 'Member Number',  // custom field label
			'size'    => 20,               // input size
			'profile' => 'only_admin',     // show in user profile for only admins
		)
	);

	foreach ( $fields as $field ) {
		pmprorh_add_registration_field(
			'just_profile', // location on checkout page
			$field          // PMProRH_Field object
		);
	}
	//that's it. see the PMPro Register Helper readme for more information and examples.
}

/**
 * https://gist.github.com/andrewlimaza/a89d4ab1790d573dd0419731c066dce3
 * This will show Register Helper fields or any user meta on the user's profile only in a read-only format.
 * (Admins won't see these fields).
 * If Register Helper is used, you may set the field's "profile" attribute
 * to be "only_admin" to make the fields editable by admins only.
 */

add_action( 'show_user_profile', 'my_pmpro_show_read_only_register_helper_field', 15, 1 );
function my_pmpro_show_read_only_register_helper_field( $user ) {
	$meta = get_user_meta( $user->ID, 'member_number', true ); //match this to your Register Helper field name.
	if ( ! empty( $meta ) ) {
		?>
		<table class="form-table">
			<tr>
				<th><?php _e( 'Member Number: ' ); ?></th>
				<td><?php echo $meta; ?></td>
			</tr>
		</table>
		<?php
	}
}

/**
 * Inspired by: https://gist.github.com/andrewlimaza/46a3e543e5d6d069e4715c6fed679e81
 *
 * This will show a member number on the membership card for Paid Memberships Pro. Please read all comments below!
 *
 * Requires custom membership card template with hooks
 * (a) pmpro_membership_card_additional_fields_top
 * (b) pmpro_membership_card_additional_fields_bottom
 * https://gist.github.com/ipokkel/c3a93a5718aa5646b80adff814ed76d4
 *
 * Add this code to your PMPro Customizations Plugin
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
*/

// to display membership number at the bottom of the card hook into 'pmpro_membership_card_additional_fields_bottom'
add_action( 'pmpro_membership_card_additional_fields_top', 'my_show_member_number_on_card' );
function my_show_member_number_on_card( $user ) {
	if ( ! pmpro_hasMembershipLevel() ) {
		return;
	}
	$member_id = get_user_meta( $user->ID, 'member_number', true );

	if ( empty( $member_id ) ) {
		// lets generate it on the fly.
		$member_id = generate_member_number( $user->ID );
	}

	echo '<p><strong>Membership Number:</strong> ' . $member_id . '</p>';
}

I have implemented this code via the Snippets plugin, but it does not appear to be working as the membership number for new signups is still the default PMPro sequential number.