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Set Theory Union of arrays in PHP

Posted by: admin April 23, 2020 Leave a comment


There are 3 operations with sets in mathematics: intersection, difference and union (unification). In PHP we can do this operations with arrays:

  • intersection: array_intersect
  • difference: array_diff

What function is for union?

No duplicates can be in the result array (like array_intersect and array_diff).

If indexes are numeric then array_merge will not overwrite the original value, but will be appended (PHP docs).

How to&Answers:

Try array_merge:

array_unique(array_merge($array1, $array2));

PHP Manual


array_unique( array_merge( ... ) )


Adrien’s answer won’t necessary produce a sequentially numbered array from two sequentially numbered arrays – here are some options that will:

array_values(array_unique(array_merge($array1, $array2)));

(Adrien’s answer with renumbering the keys afterward)


(Put the values in the keys, and use the array union operator)

array_merge($array1, array_diff($array2, $array1))

(Remove the shared values from the second array before merging)

Benchmark results (for merging two arrays of length 1000 a thousand times on my system):

  • Unique (Adrien’s version): 2.862163066864 seconds
  • Values_Unique: 3.12 seconds
  • Keys_Flip: 2.34 seconds
  • Merge_Diff: 2.64 seconds

Same test, but with the two arrays being very similar (at least 80% duplicate):

  • Unique (Adrien’s version): 2.92 seconds
  • Values_Unique: 3.15 seconds
  • Keys_Flip: 1.84 seconds
  • Merge_Diff: 2.36 seconds

It seems using the array union operator to do the actual union is the fastest method. Note however that array_flip is only safe if the array’s values are all strings or all integers; if you have to produce the union of an array of objects, I recommend the version with array_merge and array_diff.


Use array_unique and array_merge together.


use “+” operator to do so. See the link Array Operators


From the code in the PHP: Array Operators documentation:

$a = array("a" => "apple", "b" => "banana");
$b = array("a" => "pear", "b" => "strawberry", "c" => "cherry");

$c = $a + $b; // Union of $a and $b
echo "Union of $a and $b: \n";

$c = $b + $a; // Union of $b and $a
echo "Union of $b and $a: \n";

When executed, this script will print the following:

Union of $a and $b:
array(3) {
  string(5) "apple"
  string(6) "banana"
  string(6) "cherry"
Union of $b and $a:
array(3) {
  string(4) "pear"
  string(10) "strawberry"
  string(6) "cherry"


$result = array_merge_recursive($first, $second);

can be useful when you have arrays with arrays inside.


The + operator:

$x[0] = 4;
$x[1] = 1;

$y[0] = 9;
$y[2] = 5;

$u = $y + $x;

// Results in:
$u[0] === 9;
$u[1] === 1;
$u[2] === 5;

Note that $x + $y != $y + $x


The OP did not specify whether the union is by value or by key and PHP has array_merge for merging values and + for merging the keys. The results depends on whether the array is indexed or keyed and which array comes first.

$a = ['a', 'b'];
$b = ['b', 'c'];

$c = ['a' => 'A',  'b' => 'B'];
‌‌$d = ['a' => 'AA', 'c' => 'C'];

Indexed array

See array_merge

By value using array_merge

‌‌array_merge($a, $b); // [‌0 => 'a', 1 => 'b', 2 => 'b', 3 => 'c']
‌‌array_merge($b, $a); // ‌[0 => 'b', 1 => 'c', 2 => 'a', 3 => 'b']

merge by key using + operator

See + operator

‌‌$a + $b; ‌// [0 => 'a', 1 => 'b']
$b + $a; // [0 => 'b', 1 => 'c']

Keyed array

By value using array_merge

‌array_merge($c, $d); // ‌['a' => 'AA', 'b' => 'B', 'c' => 'C']
array_merge($d, $c); // ['a' => 'A',  'c' => 'C', 'b' => 'B']

merge by key using + operator

$c + $d; // [‌'a' => 'A',  'b' => 'B', 'c' => 'C']
‌‌$d + $c; // ‌['a' => 'AA', 'c' => 'C', 'b' => 'B']


Just use $array1 + $array2
It will result union of both array.