"random" characters being passed to plugin

Hi,

I’m writing a plugin, and I want to be able to pass the category belonging to content which is being viewed. I’ve tried several ways all of which pass a string looking like M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr

I’ve tried doing this four different ways each having same results:

{exp:my_plugin:events category="{exp:channel:entries}{categories}{category_name},{/categories}{/exp:channel:entries}"}

For the following two items I can use the tag {page_categories} else where on the page and will see the actual names of the categories

{preload_replace:page_categories="{exp:channel:entries}{categories}{category_name},{/categories}{/exp:channel:entries}"}
{exp:my_plugin:events category="{my_categories}"}

<?php
$categories = "{exp:channel:entries}{categories}{category_name},{/categories}{/exp:channel:entries}";
echo '{preload_replace:page_categories="'.$categories.'"}';
?>
{exp:my_plugin:events category="{my_categories}"}

For this last example if i <?php echo $categories;?> anywhere else on the page I get the category names

<?php
$categories = "{exp:channel:entries}{categories}{category_name},{/categories}{/exp:channel:entries}";
?>
{exp:my_plugin:events category="<?php echo $categories;?>"}

Does anyone have any ideas why I might be seeing the correct data on the page, but the strange string is being passed to my plugin, or have any other suggestions on how I might achieve the same goal?