$A = array (
   'porn', 'pics', 'gifs'
);

$B = array (
   'porn', 'movs', 'avi'
);

function merge_into( &$C, $a){
   $p =& $C;
   foreach ($a as $v){
      if (!array_key_exists($v,$p)) $p[$v] = array();
      $p =& $p[$v];
   }
}

$C = array();
merge_into($C,$A);
merge_into($C,$B);

print_r($C);

$a = [
    ['porn', 'pics',  'gifs', 1],
    ['porn', 'pics',  'jpg', 2],
    ['porn', 'pics',  'jpg', 3],
    ['porn', 'movs',  'flv', 4],
    ['porn', 'movs',  'avi', 5],
    ['not porn', 'pics',  'gifs', 6]
];

$b = [];

foreach($a as $c) {
    if( ! isset($b[$c[0]]))
        $b[$c[0]] = [];
    if( ! isset($b[$c[0]][$c[1]]))
        $b[$c[0]][$c[1]] = [];
    if( ! isset($b[$c[0]][$c[1]][$c[2]]))
        $b[$c[0]][$c[1]][$c[2]] = [];
    $b[$c[0]][$c[1]][$c[2]][] = $c[3];
}

echo '<pre>'.print_r($b, 1).'</pre>';

/*
$b = [
    'porn' => [
        'pics' => [
            'gifs' => [1]
            'jpg' => [2, 3]
        ],
        'movs' => [
            'flv' => [4]
            'avi' => [5]
        ]
    ],
    'not porn' => [
        'pics' => [
            'gifs' => [6]
        ]
    ]
]
*/