If you wish to execute custom PHP code in your invoice template (ie. have barcode render script added to the invoice) you can do so by adding smarty {php} tag to your invoice template source.
1. Ensure First you need to ensure your smarty have PHP handling enabled (read related article: Integrating templates - tips and tricks)
2. Go Then go to Settings → General Setting → Billing → Invoice Document Templates → Invoice customization and edit your current invoice template. Click on [HTML] button to edit source.
3. Add 
Then you can add your php code within {php} {/php} smarty tags, as below:
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