PHP array_push adding null instead of integer

From PHP documentation (http://php.net/manual/en/function.array-push.php):

Note: array_push() will raise a warning if the first argument is not an array. This differs from the $var[] behaviour where a new array is created.

The array was not initialized when the push was called.

Use $output[$ind][] = $data[$i];

Turn on error reporting while developing (I suggest to set it to E_ALL), you'll see all the problems. You're trying to push values to non existing array - you have to check if array has been already created at index you want to push and create it if it does not exists. Additional problem in your code is that you're doing one loop too much (should be $i < $length) :

$output = Array();
$length = count($data);
for ($i = 0; $i < $length; $i++) {
    $ind = $i - ($i % 2);
    if (!isset($output[$ind])) $output[$ind] = array();
    array_push($output[$ind], $data[$i]);
}

$output array is not initialized when using array_push(), try this

$data = array(10,2,123,4,34,6);
$output = Array();
$length = count($data);
for ($i = 0; $i < $length; $i++) {
    $ind = $i - ($i % 2);
    //array_push($output[$ind], $data[$i]);

    $output[$ind] = $data[$i];
}
var_dump(array_values($output));

Output

array(3) {
  [0] => int(2)
  [1] => int(4)
  [2] => int(6)
}

An easier way to accomplish the same thing:

$output = array_chunk($data, 2);