Correct syntax for a variable inside a quotes: MP4Box command

[JonnyDriller]

New to the forum so hope I'm using the correct method here: -
I'm using an app to covert h264 video to mp4 = MP4Box.
Most of the tutorials deal with conversion inside the current folder so that the command is: -

command = "MP4Box -add example.h264 example.mp4" 

This works great
Now I've found out through my own experimentation you can do something like this

command = "MP4Box -add ''/var/www/html/LifeSaverHTML/eg,h264' '/var/www/html/LifeSaverHTML/eg.mp4'"

But what I really want to do is use a variable for the path. This is my function below:-

first = path
    last = "/examplevid.mp4"    
    fullpath = first + last
    tester = "/var/www/html/LifeSaverHTML/Details/19/examplevid.h264"
    command = "MP4Box -add '"%tester"'%tester '/var/www/html/LifeSaverHTML/Details/19/examplevid.mp4'"

I've tried all sorts of suggested methods with this: -

"tester"
"'+tester+'"

I can't even remember some of the suggestions I've tried.
Can someone please tell me the correct syntax? I've been at this for 2 hours now :(