How to count the number of repeated digits in an array, php?

Question

Arrays

Hello. Sorry for the possibly stupid question.
Started learning php.
There is an array with numbers, you need to find how many times each number occurs in this array.
Decided on this.

$array_n = array(1, 2, 3, 4, 5, 2, 4, 3, 5, 1, 2, 4, 5, 3, 5, 5, 3, 1);
$ac = array_count_values ($array_n);
print_r ($ac);

But it is also necessary to find out how many times the numbers 1 and 2, 1 and 3, 1 and 4, 1 and 5, 2 and 3, etc. occur in this array.
In this version, the numbers 1 and 2 occur 2 times, 3 and 4 - 3 times. - Is there a function like array_count_values ​​that will count the number of pairs of duplicate characters?