I am using Bruce sh highlight for PHP.

Two important similarities are:
$dollar variables   and
#hash comments.

I wonder if would be easier to extend C to PHP or sh to PHP because these 3 have much in common.

PHP would be a mix of sh and C…

 I'd say NO to extending anything.

do a PHP highlight for php and the same for everything else, they may be similar but they are different and would be better to handle independently.

PS. not sure i can take credit for the sh highlight now.

I wrote the original sh highlight for gb.eval.highlight that was all gambas code and nothing to do with RegExp.
/comp/gb.eval.highlight - Gambas Documentation
comp/src/gb.eval.highlight/.src/TextHighlighter_Sh.class · master · Gambas / gambas · GitLab


Then Ben converted to the new method with gb.highlight and left it to me to convert the existing sh to the new format.  

He then had to change a few things to make it function correctly. my knowledge of gambas is much better than my knowledge of RegExp.

So the current sh highlighting is more "based on my original code" but it's not really mine any more.

Congrats for your code and humility

I said that based on a post you did in past when you uploaded to github