<?php
 
$input = '1ab2v3g4de';
$numerics = [];
 
for ($i = 0; $i < $length; ++$i) {
        $numerics[] = [$i => $input[$i]];
    }
}
 
$numLastIndex = \
count($numerics) - 1;  
foreach ($numerics as $key => $numericData) {
    $newKey = $numLastIndex - $key;
    $input[\
key($numericData)] = $numerics[$newKey][\
key($numerics[$newKey])]; }
 
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