Editing Higher-order function (section)

====PHP====
{{further information|PHP}}

<syntaxhighlight lang="php">
<?php

declare(strict_types=1);

function twice(callable $f): Closure {
    return function (int $x) use ($f): int {
        return $f($f($x));
    };
}

function plusThree(int $i): int {
    return $i + 3;
}

$g = twice('plusThree');

echo $g(7), "\n"; // 13
</syntaxhighlight>

or with all functions in variables:

<syntaxhighlight lang="php">
<?php

declare(strict_types=1);

$twice = fn(callable $f): Closure => fn(int $x): int => $f($f($x));

$plusThree = fn(int $i): int => $i + 3;

$g = $twice($plusThree);

echo $g(7), "\n"; // 13
</syntaxhighlight>

Note that arrow functions implicitly capture any variables that come from the parent scope,<ref>{{Cite web|title=PHP: Arrow Functions - Manual|url=https://www.php.net/manual/en/functions.arrow.php|access-date=2021-03-01|website=www.php.net}}</ref> whereas anonymous functions require the {{code|use}} keyword to do the same.