I host a MediaWiki website on Namcheap shared hosting on public_html/example.com.
I have uploaded certsage.txt to that directory and then changed the file name from certsage.txt to certsage.php.
My problem is that I got "Your connection isn't private" when I ran in the web browser example.com/certsage.php.
What may cause "Your connection isn't private" when running certsage.php from a web browser?
Connecting over HTTP would cause that. So would using an invalid certificate.
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You're using Certsage to get a certificate. That probably means you don't have a certificate yet. Thus, the connection isn't private warning is to be expected, as that would require a certificate you don't have yet.
Your browser should be able to bypass this warning just this once to get past this catch-22. Once you've got your cert and installed it, including HTTP to HTTPS redirect, the warning should go away (probably need to clear your browsers cache to make sure).