Require an Invite Code to Register

// Code => maximum number of sign-ups (0 = unlimited).
$wpide_invite_codes = array(
    'WELCOME2026' => 0,
    'VIP-TEAM'    => 25,
);

$wpide_find_code = function ( $entered ) use ( $wpide_invite_codes ) {
    foreach ( $wpide_invite_codes as $code => $max ) {
        if ( hash_equals( strtoupper( $code ), strtoupper( $entered ) ) ) {
            return $code;
        }
    }

    return '';
};

add_action( 'register_form', function () {
    $value = isset( $_POST['wpide_invite'] ) ? sanitize_text_field( wp_unslash( $_POST['wpide_invite'] ) ) : '';
    printf( '<p><label for="wpide_invite">Invite code</label><input type="text" name="wpide_invite" id="wpide_invite" class="input" value="%s" autocomplete="off" required></p>', esc_attr( $value ) );
} );

add_filter( 'registration_errors', function ( $errors ) use ( $wpide_invite_codes, $wpide_find_code ) {
    $entered = isset( $_POST['wpide_invite'] ) ? trim( sanitize_text_field( wp_unslash( $_POST['wpide_invite'] ) ) ) : '';
    $code    = '' !== $entered ? $wpide_find_code( $entered ) : '';

    if ( ! $code ) {
        $errors->add( 'wpide_invite', '<strong>Error:</strong> Please enter a valid invite code.' );
        return $errors;
    }

    $uses = (array) get_option( 'wpide_invite_code_uses', array() );

    if ( $wpide_invite_codes[ $code ] > 0 && ( $uses[ $code ] ?? 0 ) >= $wpide_invite_codes[ $code ] ) {
        $errors->add( 'wpide_invite_used', '<strong>Error:</strong> This invite code has been used up.' );
    }

    return $errors;
} );

add_action( 'register_new_user', function ( $user_id ) use ( $wpide_find_code ) {
    $code = isset( $_POST['wpide_invite'] ) ? $wpide_find_code( trim( sanitize_text_field( wp_unslash( $_POST['wpide_invite'] ) ) ) ) : '';

    if ( $code ) {
        $uses          = (array) get_option( 'wpide_invite_code_uses', array() );
        $uses[ $code ] = ( $uses[ $code ] ?? 0 ) + 1;
        update_option( 'wpide_invite_code_uses', $uses, false );
        update_user_meta( $user_id, 'wpide_invite_code', $code );
    }
} );