I have a form where users enter an unbounded number of rows of data. They arrive at the form by entering whatever number of rows on the screen that they desire.
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The W3 spec doesn't include rules on what order a form's values should be assembled into a data set, so technically you cannot be sure. On the other hand, I've not seen a case (from numerous browsers on numerous OSes over the years) where data hasn't been supplied in source-listing-order. I've not really tested for cases when changing the default (UI generated) tabindex values.
You could always sort the array (asort) after you've received it to be sure about what order you're reading values.