wp-postcode-plugin/admin.php

<?php

function init_postode_admin()
{
    add_action("admin_menu", "local_postcodes_admin_menu", 99);
    add_action("admin_head", "local_postcodes_admin_css");
    add_action("admin_init", "local_postcodes_register_settings"); // Correct
}

function local_postcodes_admin_menu()
{
    add_submenu_page(
        "woocommerce", // Parent menu slug
        "Lokale Postcodes", // Page title
        "Lokale Postcodes", // Menu title
        "manage_options", // Capability
        "local-postcodes", // Menu slug
        "generate_admin_page", // Callback function
        "dashicons-buddicons-pm", // Optional icon URL or dashicon
    );
}

function local_postcodes_register_settings()
{
    register_setting(
        "my_plugin_options",
        "local_postcodes_values",
        "local_postcodes_input_validate",
    );
}

function generate_admin_page()
{
    ?>
    <div class="wrap">
        <h1><span class="dashicons dashicons-admin-home"></span> Lokale Postcodereeksen</h1>
        <?php settings_errors("local_postcodes_error_messages"); ?>
        <div class="info">
        <p class="info">Vul hier de lokale postcodereeksen in.</p>
        <ul>
            <li>Het formaat voor een reeks is #### - #### (voorbeeld: 5000 - 5199).</li>
            <li>Plaats één reeks op één regel.</li>
            <li>Een reeks van 1 is geldig. (voorbeeld: 5000 - 5000)</li>
            <li>Om een reeks te verwijderen. Wis de reeks en sla vervolgens de wijzigen op.</li>
        </ul>
        </div>
        <form id="postcodes_form" method="post" action="options.php">
            <?php
            settings_fields("my_plugin_options");
            do_settings_sections("my_plugin_options");
            ?>
            <textarea id="local_postcodes_values" name="local_postcodes_values"><?php echo esc_textarea(
                get_option("local_postcodes_values"),
            ); ?></textarea>
            <?php submit_button(); ?>
        </form>
    </div>
    <?php
}

function local_postcodes_admin_css()
{
    ?>
    <style>
        h1 .dashicons{
            font-size: 25px;

        }

        .info {
            font-size: 1.1rem;
            max-width: 60ch;

            ul {
                list-style: square inside;
            }
        }

        #postcodes_form {
            display: flex;
            flex-direction: column;
            max-width: 60ch;
            align-items: flex-end;
            font-size: 1.1rem;

            #local_postcodes_values{
                width: 100% !important;
                height: 50vh !important;
                font-size: 1.1rem;
                font-family: "Consolas", "Monaco", "Courier New", monospace;
            }


            .submit{
                margin-top: .2rem;
            }
        }
    </style>';
    <?php
}

function local_postcodes_input_validate($input)
{
    $valid_input = "";
    $lines = preg_split("/\R/", $input, -1, PREG_SPLIT_NO_EMPTY);
    $pattern = '/^\d{4}\s*\-\s*\d{4}$/';
    $pc_arr = [];

    foreach ($lines as $line) {
        $line = trim($line);
        if (preg_match($pattern, $line)) {
            $parts = explode("-", $line);
            $first = (int) $parts[0];
            $second = (int) $parts[1];

            if ($first <= $second) {
                $pc_arr[] = [$first, $second];
            } else {
                $pc_arr[] = [$second, $first];
            }
        } else {
            add_settings_error(
                "local_postcodes_error_messages",
                "invalid_postcode_" . md5($line),
                '❌️ Ongeldige regel: "' .
                    esc_html($line) .
                    '". Gebruik het formaat #### - ####.(bijvoorbeeld: 5000 - 5199)',
                "error",
            );
        }
    }

    usort($pc_arr, function ($a, $b) {
        return $a[0] <=> $b[0];
    });

    $old_pc_arr_len = count($pc_arr);
    $pc_arr = array_map(
        "unserialize",
        array_unique(array_map("serialize", $pc_arr)),
    );
    $new_pc_arr_len = count($pc_arr);

    if ($new_pc_arr_len < $old_pc_arr_len) {
        add_settings_error(
            "local_postcodes_error_messages",
            "Duplicaten",
            // @formatter:off
            "⚠️ " .
                $old_pc_arr_len -
                $new_pc_arr_len .
                " " .
                ($old_pc_arr_len - $new_pc_arr_len > 1
                    ? "duplicaten"
                    : "duplicaat") .
                " verwijderd.",
            // @formatter:on
            "warning",
        );
    }

    foreach ($pc_arr as $range) {
        $valid_input .= $range[0] . " - " . $range[1] . "\n";
    }

    return $valid_input;
}