Collect all unique key paths of a multidimensional array

Question

[
    'a' => [
        'a' => ['a' => null, 'b' => null],
        'b' => ['a' => null, 'b' => null]
    ],
    'b' => [
        'a' => ['a' => null, 'b' => null],
        'b' => ['a' => null, 'b' => null]
    ]
]

How can I get the below string from the above array?

aaa,aab,aba,abb,baa,bab,bba,bbb

Answer 1

 $str = array();
foreach($array as $key1 => $value1)
{
   foreach($value1 as $key2 => $value2)
   {
     foreach($value2 as $key3 => $value3)
        $str[]= $key1.$key2.$key3;
   }
}

echo implode(',', $str);