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

The process is illustrated below:




The prototype of the `filter()` method is shown below:

```php
Ds\Vector public Ds\Vector::filter( $callback )
```
<h2>Parameters</h2>

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

<h2>Return value</h2>

The `filter()` method returns a new vector that contains all the values of the original vector 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 the following [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.

<h2>Example</h2>

The code below shows how the `filter()` method works in PHP:

<?php

// initialize vectors
$firstVector = new \Ds\Vector([1, 5, 8, 10, 15]);
$secondVector = new \Ds\Vector([0, 'abc', 10, true, false]);

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

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

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

?>

The above code produces the following output:

```php
Ds\Vector Object
(
[0] => 5
[1] => 10
[2] => 15
)
Ds\Vector 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.

<h2>Explanation</h2>

First, two vectors are initialized: `firstVector` and `secondVector`.

The `filter()` method in line $8$ proceeds to filter out values from `firstVector`. The callback function provided to this `filter()` method returns `true` if a value is divisible by $5$. Therefore, the `filter()` method returns a new vector that contains only the values of `firstVector` that are divisible by $5$.

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

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

The filter() method in PHP's DS\Vector class filters out values not meeting a callback function's condition. It returns a new vector with the valid values.