Because inside of double quotes is not possible i do it this way:
<button id=<php> echo "trtmntacc-header-" . get_row_index(); </php> aria-expanded="false" aria-controls=<php> echo "trtmntacc-panel-" . get_row_index(); </php> data-accordion-header class="accordion__header">My Button</button>
Is there a better way?
For now i am using a PHP Smart Action echoing the complete <button ....> and then again a PHP Smart Action for the </button>.
It works but is uncomfortable and makes it impossible to change the button attributes inside of the PG Panels.
I am sure, there is a better solution. Right @Emmanuel @matjaz ?
Okay… after staying away some time and come back to this problem here’s how it works:
<button id="<?php echo 'trtmntacc-header-' . get_row_index(); ?>">My Button Text</button>
Was just a matter of single and double quotes… phew… 
@MichelyWeb congrats for figuring it out! 
Another way would be:
<button id="trtmntacc-header-<?php echo get_row_index(); ?>">Hello</button>
This avoids having to use nested quotes.
Right. It’s better doing it that way.
I’m sure i tried all of this in the beginning and couldn’t get it to work. Therefore this complicated detour to the obvious solution.