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PHP autovivification
$test['a']['b'] = 1;
$test['a']['c'] = 1;
$test['b']['b'] = 1;
$test['b']['c'] = 1;

var_dump($test);

array
  'a' =>
    array
      'b' => int 1
      'c' => int 1
  'b' =>
    array
      'b' => int 1
      'c' => int 1

$test['a'][4] = 1;
$test['b'][4]['f'] = 3;

$test['a'][4]['f'] = 3;

$test['a'][4]['a'] = 1;
$test['a'][4]['b'] = 2;
$test['a'][5]['c'] = 3;
$test['a'][8]['d'] = 4;

$test['a'][4] = 1;

$test['b'][4]['f'] = 3;

$test['a'][4]['f'] = 3;

$test['b'][4]['f'] = 3;

$test['a'][4]

$test['a'][4] = array(1);

$test['a']['b'] = 1;
$test['a']['c'] = 1;
$test['b']['b'] = 1;
$test['b']['c'] = 1;