How to classify the multidimensional array by value?

Question

How can I classify this array for the value of the key level, even though the values are currently sequential, they will not always be?

array:67 [▼
  0 => array:3 [▼
    0 => array:8 [▼
      "idPai" => 1
      "id" => 93
      "nome" => "Pesoa"
      "nivel" => 2
      "email" => "[email protected]"
      "telefone" => "fone"
      "endereco" => "endereco"
      "imagem" => "uploads/avatars/1547216218.jpeg"
    ]
    1 => array:8 [▶]
    2 => array:8 [▶]
  ]
  1 => array:3 [▼
    0 => array:8 [▼
      "idPai" => 93,
      "id" => 96,
      "nome" => "Pesoa",
      "nivel" => 2,
      "email" => "[email protected]",
      "telefone" => "fone",
      "endereco" => "endereco",
      "imagem" => "uploads/avatars/1547216218.jpeg"
    ]
    1 => array:8 [▶]
    2 => array:8 [▶]
  ] 
3 => array:3 [▶]
  4 => []
  5 => []
  6 => []

I would first like to filter these records and ensure that the sequence of levels be followed, example, all the levels 1, then all the levels 2, then all the levels 3 and so it goes.

I tried to

usort($array, function ($a, $b){
    if (count($a) > 0 and count($b) > 0) {
      return strcmp($a["nivel"], $b["nivel"]);
    }
});

But it didn’t work, as I can do to sort based on the value of the index level these guys?

The method that mounts the array is this

public function getMatrizRecursive($users = null, $nivel = 1)
{
$nivel++;

if ($nivel > 9) {
  return $this->matriz;
}

$montaArrayRede = [];

foreach ($users as $p){

  $children = User::where('id', '=', $p->posicao1)
  ->orWhere('id', '=', $p->posicao2)
  ->orWhere('id', '=', $p->posicao3)->get();

  foreach($children as $pessoa){
    $montaArrayRede[] = [
    'idPai' => $p->id,
    'id' => $pessoa->id,
    'nome' => $pessoa->nome,
    'nivel' => $nivel,
    'email' => $pessoa->email,
    'telefone' => $pessoa->telefone,
    'endereco' => $pessoa->cidade,
    'imagem' => $pessoa->logo_number
    ];
  }
  array_push($this->matriz, $montaArrayRede);

  $this->getMatrizRecursive($children, $nivel);
}

return $this->matriz;

 }

Note: some indexes return empty.