In PHP, the **`Ds\Set::union()`** **function** is used to find the union of two sets. The union of two sets is a set that contains all the elements that are present either in the first set or the second set.

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## Syntax
The `Ds\Set::union()` function can be declared as shown in the code snippet below:
```PHP
Ds\Set public Ds\Set::union ( Ds\Set $set )
```

## Parameter
The `Ds\Set::union()` takes the following parameter:
* **`set`**: This is the second set that takes union with the first set.

## Return value
The `Ds\Set::union()` function returns the union of the sets, that is, a set containing the elements that are present in the first or second set.

## Code example
Let's look at the code snippet given below, which demonstrates the use of the `Ds\Set::union()` function:

<?php

$set1 = new \Ds\Set([1, 2, 3, 4]);

$set2 = new \Ds\Set([3, 4, 5, 6]);

echo("Union of set1 and set2: \n");

print_r($set1->union($set2));

?>

phpDs

## Explanation
* Line 3: We declare a set called `set1`.
* Line 5: We declare a set called `set2`. 
* Line 9: We call the `Ds\Set::union()` function to find the union of `set1` and `set2`.

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What is the Ds\Set union() function in PHP?

The Ds\Set::union() function in PHP combines two sets, returning a set with elements from both.