PHP on CMS across all themes in Magento

I'm new to Magento and I've been converting an existing bespoke web site to Magento.

I have occasional PHP includes on the existing site that need to be inserted into CMS pages on the new site. I've managed to do this by adding blocks that call a phtml file within the layouts directory. However, I would like to make sure that the PHP include is available to all themes. The way I've done it means that it's only easily available to the theme I'm using - I need to develop a mobile theme and use the same PHP includes.

I've spent some time looking at modules and widgets. I finally got my head around widgets but now seem unable to insert PHP into it - it seems it only supports HTML so I'm back to square one.

Can someone please explain what the best practice is and possibly expand on that so I can implement it in my site?
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Vimal DMSenior Software EngineerCommented:

You can write your own PHP code in PHTML, it will work same as php file in magento, you can just copy and paste what ever you have in php file to phtml and fixing if there is any issues.
edz_pgtAuthor Commented:
Yet again, I've ended up answering my own question in the absence of a quality EE contribution.

The includes need to be located in the default/default directory structure rather than the default/theme directory. This way, all themes will see the files.

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edz_pgtAuthor Commented:
I seem to be struggling to get quality responses from EE these days. Not too sure whether continuing my subscription is of any value.
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