Googlebot never makes up URLs. It crawls URLs because it finds links to them. If Google is crawling a URL, it must be linked from somewhere on the web.
In this case, one bad link into a page with an extra trailing slash probably caused your site to show additional broken links to Googlebot.
By default, web servers and PHP allow any additional path to be appended to a PHP file and it will show the same content as the original PHP file. So on your server all these URLs probably show the contents of
You could fix this by putting a rewrite rule at the top of
.htaccess to redirect away from the added path:
RewriteRule (.*\.php).+ /$1 [R=301,L]
Before you do that, you should take a look at the page you serve when the URL is not what you expect. So if somehow a link got created to
/index.php/, it seems likely that all the links on the page are broken. You are probably linking with relative URLs like
<a href="about_us.php">About Us</a>. From
/index.php that will work fine, but from
/index.php/ the link doesn't go where you expect.
I recommend changing all your links to "root relative" links that begin with a slash. Doing so prevents problems such as this because the links are relative to your site's domain name, and no longer relative to directory they are in. So I would recommend changing all your links to add a slash at the beginning like
<a href="/about_us.php">About Us</a>. With root relative linking, even if Googlebot finds one malformed URL, it isn't going to propogate the problem to every link on the page.