Is this code likely to stand the test of time? (Add select box to DOM with JS)
To answer your question in the title: no
For multiple reasons. One being that the things we use today could be out-dated in a few years. who knows
One question/remark:
sel_obj.options[i]=new Option(op_array[i], i);
sel_obj.options[i].value=op_array[i];

is there a reason why you are setting the value twice? (once in the new Option call and once on the next rule)
My suggestions are:
Separation of concerns
You have one file with html, javascript and php all interweaved with each other. So debugging won't make it any easier. Alsog reusing a part of that script is nearly impossible since everything is hard coded.
Overuse of javascript
Is there a reason why you are building the html with javascript? Because this has some drawbacks: Screenreaders will not be able to see the selectlist items since they don't parse javascript. Serach engines will not know about the select-options.
Instead you should use PHP to create the html for the select items.
Hard to read code
The code is very hard to read, the function and variable names don't clearly tell what they are or have typehinting in the names. For instance op_array variable. Words as Array, String, int,.. should  be banned from variable names. Simply call it options. Now by simply reading it I allready know that is has multiple values and that it are the options. Where as op_array tells me that it is an array of 'op'.
Also for future enhancements. If for some reason you are going to change the way options are passed in, you will also have to change the name of the variable...
Also be more consistent in your naming. populateList() needs a select and options. But no list. This is confusing.
Personally I never use the onClick attributes inhtml. Not because they are bad. But because if you do use them, javascript will be hidden in the html and tightly coupled with it whereas if you EvntListners it is very easy to change the html without having to bother about the javascript. this is also better for sepperation of concerns