Array_diff in foreach?

I need to compare values from some arrays. This array is multi dimensional and I need to compare the arrays inside.

Here the dump:

php

    array (size=4)
  1 => 
    array (size=3)
      0 => string '96' (length=2)
      1 => string '90' (length=2)
      2 => string '91' (length=2)
  2 => 
    array (size=3)
      0 => string '96' (length=2)
      1 => string '90' (length=2)
      2 => string '91' (length=2)
  3 => 
    array (size=4)
      0 => string '96' (length=2)
      1 => string '90' (length=2)
      2 => string '91' (length=2)
      3 => string '98' (length=2)
  4 => 
    array (size=4)
      0 => string '96' (length=2)
      1 => string '90' (length=2)
      2 => string '91' (length=2)
      3 => string '98' (length=2)

I wanted to use something like array_diff, to compare the different arrays but... even if it seems stupid, I don't know how to do it. I guess I expect to "extract" the 4 array, to be able to compare them.

Is there somebody that can explain me a good way to do this ? Thank you very much.

<?php

$arr = [
    ['96','90','91'],
    ['96','90','91'],
    ['96','90','91','98'],
    ['96','90','91','98'],
];

$set = [];

foreach ($arr as $values) {
    foreach($values as $each_value){
        if(!isset($set[$each_value])) $set[$each_value] = true;
    }
}

$result = [];

$set = array_keys($set);

foreach ($arr as $values) {
    foreach($set as $value){
        if(!in_array($value,$values)) $result[] = $value;
    }
}

$result = array_unique($result);

print_r($result);

Demo: https://3v4l.org/gMZUR

We first make a set of elements to be present across each array.
Later, we move across all arrays once again and check each element in the set against each array values.
If we find an element in the set not present in any of the each individual sub arrays, we add it in the result.
Note that we can count frequency of element and compare it with the size of the actual data set, but duplicate values could cause issues.