<?php $array = [1, 1, 2, 3, 4, 5, 6, 7, 9, 9, 10]; $result = []; foreach ($array as $index => $value) { if (!isset($result[$value])) { $result[$value] = $index; } else { $result[$value] .= ",$index"; } } var_export(array_values($result));
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