Add a global custom field

global-custom-field.php

 // Add this to your functions.php:
 <?php add_action('admin_menu', 'add_gcf_interface');
   function add_gcf_interface() {
        add_options_page('Global Custom Fields', 'Global Custom Fields', '8', 'functions',
       'editglobalcustom elds');
   }
   function editglobalcustom elds() { ?>
        <div class="wrap">
               <h2>Global Custom Fields</h2>
               <form method="post" action="options.php">
               <?php wp_nonce_ eld('update-options') ?>
                       <p><strong>Amazon ID:</strong><br />
                              <input type="text" name="amazonid" size="45"
                              value="<?php echo get_option('amazonid'); ?>" />
                      </p>
                       <p><input type="submit" name="Submit" value="Update Options" /></p>
<input type="hidden" name="action" value="update" />
</div> </form> <input type="hidden" name="page_options" value="amazonid" />
<?php } ?>

// You can now display this value anywhere in your theme with the get_option() template tag:
   <?php echo get_option('amazonid'); ?>