The callable type

PHP course · Expert · Lesson 11/54

Learn to make a function accept another function with the callable type declaration.

The callable type declaration

You can pass one function as a parameter to another. For this you use the callable type declaration.

Example: function apply(callable $cb, int $x): int { return $cb($x); }. Here $cb is a function; it is called with $cb($x).

callable accepts three forms: a function name string ('strtoupper'), an anonymous function, or a ['Class', 'method'] array. This flexibility makes writing callbacks easy.

Check with is_callable

Sometimes you need to know in advance whether a value is callable. is_callable($x) does this check; it returns true or false.

It is useful to validate a callback name coming from the user. is_callable('strtoupper') returns true; is_callable('nope') returns false.

If a wrong-typed value is passed to a callable parameter, PHP throws a TypeError; is_callable prevents that.

Exercise Define the apply function

Define a function apply(callable $cb, $x) that returns $cb($x). Then call apply(fn($n) => $n * 10, 5) and print the result on a new line.

Show example solution

Write function apply(callable $cb, $x) { return $cb($x); }. Then call echo apply(fn($n) => $n * 10, 5) . "\n";.
Example solution:
<?php
// Define apply and call it with an arrow function
function apply(callable $cb, $x) {
    return $cb($x);
}
echo apply(fn($n) => $n * 10, 5) . "\n";

Test yourself

Complete the callable type declaration

<?php
function apply(____ $cb, $x) {
    return $cb($x);
}
echo apply(fn($n) => $n * 10, 5) . "\n";

Show answer

<?php
function apply(callable $cb, $x) {
    return $cb($x);
}
echo apply(fn($n) => $n * 10, 5) . "\n";

Which is NOT a valid callable form?
- 'strlen'
- ['Foo', 'bar']
- function() {}
- [1, 2, 3]
Show answer

[1, 2, 3] — Correct. [1, 2, 3] is an array of numbers; it is not callable.
Match the callable forms

'strlen'

Name string
function() {}

Anonymous function
['Foo', 'bar']

Class method array
is_callable()

Checks if callable

All lessons in Expert

The callable type