You need to use Terminal command line. You can filter by filename with asterisk wildcard or other options with ls command, or use grep command to match things inside the files you want.

Terminal. ls for starters. You could do

ls -al {path to folder} > folder_contents.txt

Replace {path to folder} with the directory’s path. I like to drag and drop the folder from terminal into finder.

This will create a text file folder_contents.txt which you can open with a file editor to view the names of all files in that directory. Or if you don’t mind using the terminal to browse the text file I recommend less. Hope that helps!

Terminal. ls for starters. You could do

ls -al {path to folder} > folder_contents.txt

Replace {path to folder} with the directory’s path. I like to drag and drop the folder from terminal into finder.

This will create a text file folder_contents.txt which you can open with a file editor to view the names of all files in that directory. Or if you don’t mind using the terminal to browse the text file I recommend less. Hope that helps!

maybe try VSCode? it’s pretty quick with loading files and it’s free

Terminal.

With that many files, you should be using CLI tools.

BTW those aren't actual PHP files, they're most likely HTML. PHP is what they're called when they're code on the live website but the tools that make these archives only copy a dump of the webpages. It should have renamed them but apparently didn't. You'll have to do the renaming instead in order to open them. It's also extremely likely that the links between the forum pages are also ending in .php and won't work (the tool was also supposed to have converted the links inside the files).

Use an indexing tool like NeoFinder to generate a searchable offline index of the folder or volume. Then you can use the NeoFinder index to find what you want, then right-click on the file and select “Reveal in Finder”