The FILTER_CALLBACK constant calls a user defined function to filter the value.
Return
The FILTER_CALLBACK constant does not return anything.
Example
The following is an example that converts the case of a string. Here, existing function in PHP is taken.
<?php $string = "DEMO TEXT!"; echo filter_var($string, FILTER_CALLBACK,array("options" = >"strtolower")); ?>
Output
The following is the output.
demo text!