The simple answer is... its wrong 
(although you were on the right path)
$i is one element/string of the $split array
Each time through the loop you are just over writing the string $i. Nothing is really changing.
If you look at my sample above, you will see I am overwriting the value in the array, and then once all the values are recreated, creating a string with the join.
Clear as mud?
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(... better days ahead)
