The PHP callable keyword is used to force a function argument to be a reference to a function.

A callable can be one of the following:

• An anonymous function
• A string containing the name of a function
• An array describing a static class method
• An array describing an object method

<?php
function printFormatted(callable $format, $str) {
    echo $format($str);
    echo '<br>';
}

function exclaim($str) { return $str . '!'; }

printFormatted("exclaim", 'Hello World');
?>