Fill Textfield On Select From Drop Down List

I know very little javascript, I am more familiar with php. I am working with javascript code, php/mysql backend and smarty templates. I am trying to get the textfield box to update with a variable from the database based on the selection from the drop down field. My dynamic drop down selection works fine, it is pulling the data from the database (example:$item[i].PARTS_RATE_ID}) just fine. But I want the textfield ('parts_price['+iteration+']') to dynamically show the $item[i].PARTS_RATE_COST from the database (in the parts section), associated with the $item array from the "parts description".

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