How can you get unique values from multiple arrays using array_unique?

Question

Is there any way to put two or more arrays into the array_unique() function? If not, is there any other solution to get unique results from multiple arrays?

Answer 1

The answer to the first part of your question is NO.

The array_unique function definition in the PHP manual states that array_unique takes exactly two arguments, one array, and an optional integer that determines the sorting behavior of the function.

array array_unique ( array $array [, int $sort_flags = SORT_STRING ] )

Rather than take the manual's word for it, here are some test arrays.

$one_array      = ['thing', 'another_thing', 'same_thing', 'same_thing'];
$two_arrays     = ['A', 'B', 'C', 'thing', 'same_thing'];
$or_more_arrays = ['same_thing', 1, 2, 3];

A couple of test show that the function does work as advertised:

$try_it = array_unique($one_array);

returns ['thing', 'another_thing', 'same_thing'];

$try_it = array_unique($one_array, $two_arrays);

gives you a warning

Warning: array_unique() expects parameter 2 to be integer, array given

and returns null.

$try_it = array_unique($one_array, $two_arrays, $or_more_arrays);

also gives you a warning

Warning: array_unique() expects at most 2 parameters, 3 given

and returns null.

---

The answer to the second part of your question is YES.

To get unique values using array_unique, you do have to have one array of values. You can do this, as u_mulder commented, by using array_merge to combine the various input arrays into one before using array_unique.

$unique = array_unique(array_merge($one_array, $two_arrays, $or_more_arrays));

returns

['thing', 'another_thing', 'same_thing', 'A', 'B', 'C', 1, 2, 3];

If instead of several individual array variables, you have an array of arrays like this:

$multi_array_example = [
    [1, 2, 3],
    [2, 3, 4], 
    [3, 4, 5],
    [4, 5, 6]
];

Then you can unpack the outer array into array merge to flatten it before using array_unique.

$unique = array_unique(array_merge(...$multi_array_example));

Or in older PHP versions (<5.6) before argument unpacking, you can use array_reduce with array_merge.

$unique = array_unique(array_reduce($multi_array_example, 'array_merge', []));

returns [1, 2, 3, 4, 5, 6]

Answer 2

By default, array_unique() takes two parameters, the array, of type array takes the input array and the second parameter, sorting of type int takes the sorting behavior. You can read more on it in the PHP manual for array_unique().

But as suggested by @u_mulder, you merge the array first, probably into a temporary array (you can learn how to do that here) and then apply the array_unique to that temporary array.