I could show u scary ways that u can embed data in files, metadata, exif, that text editor would never see or find. Tools like VirusTotal, they have a limit of certain number of lines of code, iot skips rest without telling you, and also it will only go so deep into recursive archive. its mostly useless.
Truth is if you are going to go to the effort of learning how to scan trhough files by hand, and clean them up, then might as well be developing your own stuff, or spending that same time on making some $$ whicher way and just buying the absolute essentials and those things u would be generating profit from or using commercially. I promise you that the peace of mind alone is priceless.. Bearing in mind also that Authors/Devs are human, and right this very minute there are at least 3 mainstream php based scripts that allow for upload & execution of both php & via cgi.
And what both funny & sad about it, one of them is something i wanted and would happily pay for, but has shells injected everywhere as the dumbass author uses demo server as dev as well.
And that goes back to the Virus Scanning, If Envato cant pick up shells injected into that script, and being
distributed by their file provisioning / download server, then things like VT have no hope.
One quick hack you could do, is just replace all non php based code, ie : images, css, js, html, vue, and so on, with the files from their demo, if u lucky they are the same as distribution, but occasionally it will have some featuers nerfed..
Another quicky is if there is an included package.json then just replace all vendor packages completely. or at least compare them, u will find that more than a few authors, hide code within some random vendor code.
That all said, envato themselves have seirously dropped theball, there has been virtually zero moderation on their part of submissions.. Guys are literally running around with cocks out flapping, and uploading same script/theme to 4 products with minor changes in css.. MINOR. Cant even trust legit purchases really,