I would highly NOT recommend to write a software in PHP, that has access to /bin or other relevant system parts and serve it via apache. Because: PHP is not strict, reliable and secure enough to realize programs as Webmin with.
But I wonder at all, why a manager would want to do administrative tasks on a server? Are those really administrative tasks? Or is it something, you could write in PHP from scratch? I mean, If he just wants to shrink an image, you don't need a bash script for that.
I wrote a remote installer in python once, that was only accessible in the companies intranet and executed a few scripts via an SSH connection, using certificates. It worked, but was really not a good solution. But I would recommend a way like this one, to have a relatively secure way to hack this together.