Add custom field in the admin area of a WordPress dashboard

Following snippet will add AS Ads Manager textarea field in the WordPress Dashboard.

// create admin area fields
add_action('admin_menu', 'add_global_custom_options');
function add_global_custom_options()  
{  
    add_options_page('AS Ads Manager', 'AS Ads Manager', 'manage_options', 'functions','global_custom_options');  
}

function global_custom_options()
{
?>
    <div class="wrap">
        <h2>AS Ads Manager</h2>
        <form method="post" action="options.php">
            <?php wp_nonce_field('update-options') ?>
            <p><strong>Enter Your Ad Code Here:</strong><br />
                <textarea name="as-job-ads" style="width:100%;height:300px" ><?php echo get_option('as-job-ads'); ?></textarea>
            </p>
            <p><input type="submit" name="Submit" value="Update Ad Code" /></p>
            <input type="hidden" name="action" value="update" />
            <input type="hidden" name="page_options" value="as-job-ads" />
        </form>
    </div>
<?php
}

Now If you want to display the content of that textarea in the frontend then you can use the following snippet in your theme’s template files(archive, single post, etc).

<!-- AS Ads-->
<?php echo get_option('as-job-ads') ?>
<!-- AS Ads-->