Fontawesome added duotone support back in July 2019. So this is a little overdue, but better late than never! You can now use FontAwesomeSVG-PHP v1.2 to output Fontawesome duotone SVG icons with PHP (no JS required).

Output a duoetone icon in single colour:

1echo $FA->get_svg('fad fa-laugh-wink', [
2    'fill' => '#e64980',
3]);

Swap layer opacity by adding the fa-swap-opacity class:

1echo $FA->get_svg('fad fa-laugh-wink', [
2    'fill'  => '#e64980',
3    'class' => 'fa-swap-opacity',
4]);

Single colour with custom opacity:

1echo $FA->get_svg('fad fa-laugh-wink', [
2    'fill' => '#e64980',
3    'secondary' => [
4        'opacity' => '0.2',
5    ],
6]);

Custom colours and opacity:

 1echo $FA->get_svg('fad fa-laugh-wink', [
 2    'primary' => [
 3        'fill'    => '#e64980',
 4        'opacity' => '0.5',
 5    ],
 6    'secondary' => [
 7        'fill'    => '#fcc417',
 8        'opacity' => '1',
 9    ],
10]);

Head to the GitHub repo to learn more.