Page Comparison

...

1. Ensure your smarty have PHP handling enabled: Integrating templates - tips and tricks

2. Go to Settings ->Billing->Invoice → Billing → Invoice customization and edit your current invoice template. Click on [HTML] button to edit source. 

3. Add your php code within {php} {/php} smarty tags. like, as below:

 {php} echo "Invoice generated at: ".time(); {/php}

...