Warning: in_array() expects parameter 2 to be array, null given (loop) - php

Question

I'm looping for string overlaps between two arrays, deleting those values where there is one, so that only the empty values of $check remain, in this case $check[5].
The second one, $check, is multidimensional.
$names = ["bob", "selena", "hailey", "rob", "justin", "robocop"];
$check = [
["justin"], //bob
["justin", "selena", "robocop"], //selena
["justin"], //hailey
["justin", "rob"], //rob
[], //justin
["justin", "selena", "bob"] //robocop
];
for ($i = 0; $i < count($names); $i++) {
for ($j = 0; $j < count($check); $j++) {
if (in_array($names[$i], $check[$j])) {
unset($check[$j]);
}
}
}
The first loop runs through $names, the second through $check.
If the current string from $names ($names[i])
is present in the current array of $check ($check[j])
the array is removed.
However, the console prints a warning: in_array() expects parameter 2 to be array, null given. I find this strange, because $check[j] should be equal to one of the arrays inside of $check.
Is there any way I can fix this?

Answer 1

After a few iterations of the outer loop, you will have unset most of $check. With a for loop like that, you're counting on specific numeric keys being there, but since you unset them, their values are undefined, a.k.a. null.
You can avoid this by using foreach loops instead.
foreach ($names as $name) {
foreach ($check as $key => $array) {
if (in_array($name, $array)) {
unset($check[$key]);
}
}
}

Answer 2

As you are keep on unsetting the $check array, the size of the $check array will go down. Hence the $check won't return a value for the higher values of $j
You can do a minor modification in this code segment as follows
$checkcount = count($check);
for ($i = 0; $i < count($names); $i++) {
for ($j = 0; $j < $checkcount; $j++) {
if (isset($check[$j]) && in_array($names[$i], $check[$j])) {
unset($check[$j]);
}
}