FILTER_CALLBACK constant in PHP

The FILTER_CALLBACK constant calls a user−defined function to filter the value. This constant is used with filter_var() to apply custom filtering logic by passing a callback function.

Syntax

filter_var($value, FILTER_CALLBACK, array("options" => "callback_function"));

Parameters

The callback function is specified in the options array and receives the input value as a parameter. The function should return the filtered value.

Example 1: Using Built−in Function

This example converts a string to lowercase using PHP's built−in strtolower() function ?

<?php
    $string = "DEMO TEXT!";
    echo filter_var($string, FILTER_CALLBACK, array("options" => "strtolower"));
?>

demo text!

Example 2: Using Custom Function

This example demonstrates using a custom callback function to remove spaces and convert to uppercase ?

<?php
    function customFilter($value) {
        return strtoupper(str_replace(' ', '_', $value));
    }
    
    $string = "hello world php";
    echo filter_var($string, FILTER_CALLBACK, array("options" => "customFilter"));
?>

HELLO_WORLD_PHP

Example 3: Using Anonymous Function

You can also use anonymous functions as callbacks ?

<?php
    $number = "123.456";
    $rounded = filter_var($number, FILTER_CALLBACK, array(
        "options" => function($value) {
            return round((float)$value, 1);
        }
    ));
    echo $rounded;
?>

123.5

Conclusion

FILTER_CALLBACK provides flexible data filtering by allowing custom functions. It's useful when built−in filters don't meet specific requirements, enabling you to apply any transformation logic to input data.