<?php $array = [ 'a' => [ 'a' => [ 'a' => null, 'b' => null ], 'b' => [ 'a' => null, 'b' => null ] ], 'b' => [ 'a' => [ 'a' => null, 'b' => null ], 'b' => [ 'a' => null, 'b' => null ] ] ]; function getKeyPaths(array $array, string $prefix = ''): string { return implode( ',', array_map( fn($k, $v) => is_array($v) ? getKeyPaths($v, "$prefix$k") : "$prefix$k", array_keys($array), $array ) ); } var_export(getKeyPaths($array));
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