Hardcode Subs with FFMPEG - SRT & WebVTT both seem to work.

gistfile1.txt

ffmpeg -i input-without-subs.mp4 -c:v libx264 -vb 20M -c:a copy -vf subtitles=subs.srt output-with-subs.mp4

To change the font size use this one:
  ffmpeg -i input.mp4 -c:a copy -c:v libx264 -vb 20M -vf "subtitles=try_1.en_us.srt:force_style='Fontsize=24'" output-with-subs.mp4 

To do a two pass encode while hardcoding subs: 
  ffmpeg -y -i input.mp4 -vf subtitles=subs.srt -c:v libx264 -b:v 5000k -pass 1 -an -f mp4 /dev/null 
  ffmpeg -y -i input.mp4 -vf subtitles=subs.srt -c:v libx264 -b:v 5000k -pass 2 -c:a copy output.mp4

can this be used on laravel?

I don't know laravel well but in PHP you could use exec. You'd need ffmpeg installed on the server. https://www.php.net/manual/en/function.exec.php