The **`filter()` method** of the `DS\Set` class in PHP eliminates the values that fail to satisfy the condition defined in a provided callback function from a set.

The process is illustrated below.

#   
#  Syntax
The prototype of the `filter()` method is shown below.

```PHP
public Ds\Set::filter(callable $callback = ?): Ds\Set
```
##   
## Parameters

The `filter()` method accepts a single optional parameter: a callback function that defines the condition based on which values are eliminated from the set.

## Return value</h2>

The `filter()` method returns a new set that contains all the values of the original set for which the callback function returns `true`.

If the callback function is not provided as a parameter, the `filter()` method only retains the values evaluated as `true`.

> Note: You can check [this link](https://www.php.net/manual/en/language.types.boolean.php#language.types.boolean.casting) to learn how values are evaluated as `Boolean` in PHP.

## Code

The example below shows how the `filter()` method works in PHP.

<?php

// initialize sets
$firstSet = new \Ds\Set([1, 5, 8, 10, 15]);
$secondSet = new \Ds\Set([0, 'abc', 10, true, false]);

// filter out all values not divisible by 5
$filterFirst = $firstSet->filter(function($value){
  return $value % 5 == 0;
});

// filter without callback
$filterSecond = $secondSet->filter();

// print filtered sets
print_r($filterFirst);
print_r($filterSecond);

?>

#   
##   
### Output
The code above produces the following output.

```PHP
Ds\Set Object
(
[0] => 5
[1] => 10
[2] => 15
)
Ds\Set Object
(
[0] => abc
[1] => 10
[2] => 1
)
```

> Note: To run this program, you need to install [Data Structures](https://www.php.net/manual/en/book.ds.php) for PHP on your local machine.

## Explanation

First, two sets are initialized: `firstSet` and `secondSet`.

The `filter()` method in line $8$ proceeds to filter out values from `firstSet`. The callback function provided to this `filter()` method returns `true` if a value is divisible by five. Therefore, the `filter()` method returns a new set that contains only the values of `firstSet` that are divisible by five.

The `filter()` method in line $13$ filters out values from `secondSet`. Since no callback function is provided to the `filter()` method, it eliminates all the values of `secondSet` that do not evaluate to `true`, i.e., `0` and `false`.

What is the DS\Set filter() method in PHP?

The filter() method in PHP's DS\Set class removes values not meeting a callback's condition. It returns a new set preserving values satisfying the condition.