try

$array = json_decode( json_encode($array), true);

EDIT: I've tested this case, and it works:

$stdClass= new stdClass();
$stdClass->test = "foo";
$array = Array(
    "a" => Array("b","c"),
    "d" => $stdClass
);

$array = json_decode( json_encode($array), true);

var_dump($array);

OUTPUT

array
  'a' => 
    array
      0 => string 'b' (length=1)
      1 => string 'c' (length=1)
  'd' => 
    array
      'test' => string 'foo' (length=3)

This is an odd object, not going to lie.

Do either of these work? (Assuming that your print_r is a print_r($this);)

unset($this->{1});
unset($this->{'1'});

More info: Is it possible to delete an object's property in PHP?

EDIT: Though, I'd recommend changing the way you're getting this information through PDO so that it's in array format. More information on how to do this here: PDO returning execute results to an array

Once you have it in an array format, you can simply use:

unset($data[1]);

You can filter out the empty objects.

$data = array_values(array_filter($data, function($item) {
    return (bool) (array) $item;
}));

array_values is necessary to prevent the {"0":{"id":2088996538},"2":{"id":2077495673}} form you were getting, because it reindexes the array.

The intermediate cast to array in the callback is necessary because any object evaluates to true whether it's empty or not, but an empty array will evaluate to false. If you decode to arrays instead of objects by setting the second parameter of json_decode to true, that cast isn't necessary.

Not directly related to the original question, but here's my suggestion for another way to do this starting with the data from the API:

$activities = json_decode($json, true);

$set_keys = array_flip(["start_latlng", "id", "start_date","name"]);

foreach ($activities as $activity) {
    if (!is_null($activity['start_latlng'])) {
        $result[] = array_intersect_key($activity, $set_keys);
    }
}

echo json_encode($result);

Try this

$arr=array(17,10);
foreach ($array as obj) {
  if (!in_array($obj->category_id, $arr)) {
    unset($array[$key]);
  }
} 
// edited, missing closing bracket

you can try

json_decode($yourJson,true);

for force return associative array

You can try with this code:

/* you have a $collection array with all objects */

$newCollection = array();
foreach ($collection as $item) {
    if (is_array($item) && count($item)) {
        foreach ($item as $subItem) {
            $newCollection[] = $subItem;
        }
    }
}

/* $newCollection is the new array collection */

unset($a->new_property);

This works for array elements, variables, and object attributes.

Example:

$a = new stdClass();

$a->new_property = 'foo';
var_export($a);  // -> stdClass::__set_state(array('new_property' => 'foo'))

unset($a->new_property);
var_export($a);  // -> stdClass::__set_state(array())

You can do

function unsetValue(array $array, $value, $strict = TRUE)
{
    if(($key = array_search($value, $array, $strict)) !== FALSE) {
        unset($array[$key]);
    }
    return $array;
}

You can also use spl_object_hash to create a hash for the objects and use that as array key.

However, PHP also has a native Data Structure for Object collections with SplObjectStorage:

$a = new StdClass; $a->id = 1;
$b = new StdClass; $b->id = 2;
$c = new StdClass; $c->id = 3;

$storage = new SplObjectStorage;
$storage->attach($a);
$storage->attach($b);
$storage->attach($c);
echo $storage->count(); // 3

// trying to attach same object again
$storage->attach($c);
echo $storage->count(); // still 3

var_dump( $storage->contains($b) ); // TRUE
$storage->detach($b);
var_dump( $storage->contains($b) ); // FALSE

SplObjectStorage is Traversable, so you can foreach over it as well.

On a sidenote, PHP also has native interfaces for Subject and Observer.

Is unset($row); what you want? But there is no need to do that if you are just doing it to save memory...

You could use array_filter to do this:

$objects = array(
    (object)array('id' => 1, 'name' => 'test1'),
    (object)array('id' => 2, 'name' => 'test2'),
    (object)array('id' => 3, 'name' => 'test1'),
);
$known = array();
$filtered = array_filter($objects, function ($val) use (&$known) {
    $unique = !in_array($val->name, $known);
    $known[] = $val->name;
    return $unique;
});

check out this fiddle