Use array_filter with a callback:

$r = array_filter($array, function($v) { return !empty($v['fcmToken']); });

For checking exactly null:

$r = array_filter($array, function($v) { return !is_null($v['fcmToken']); });

Here we are using foreach to iterate over values and using $value for non-empty $value["fcmToken"]

$result=array();
foreach($array as $key => $value)
{
    if(!empty($value["fcmToken"]))
    {
        $result[]=$value;
    }
}

print_r($result);

Output:

Array
(
    [0] => Array
        (
            [fcmToken] => dfqVhqdqhpk
        )

    [1] => Array
        (
            [fcmToken] => dfgdfhqdqhpk
        )

)

The best and easy single line solution for filter multidimensional array like below.

$aryMain = array_filter(array_map('array_filter', $aryMain));

I hope this solution is work for you. for more detail please visit below link.

PHP: remove empty array strings in multidimensional array