Higher-order functions
Learn to write higher-order functions that return or accept functions.
What is a higher-order function?
A function can both take another function as a parameter and return a function. Such functions are called higher-order.
For example makeMultiplier(2) returns fn($n) => $n * 2. You assign the returned closure to a variable and call it later. This produces flexible tools instead of fixed code.
This pattern is already used in built-ins like array_map and array_filter; now you write your own.
Function composition
The power of higher-order functions comes from composition. The output of one function can be the input of another.
For example compose($f, $g) returns fn($x) => $f($g($x)). This applies two operations in a single step.
This pattern is the heart of functional programming; you pipe data from one operation to the next. You build complex flows from small pieces.
Exercise The makeMultiplier function
Define a function makeMultiplier(float $factor) that returns an arrow function multiplying by the given factor (fn($n) => $n * $factor). Then call $triple = makeMultiplier(3); and print the result with echo $triple(5) . "\n";.
Show example solution
Write function makeMultiplier(float $factor) { return fn($n) => $n * $factor; }. Then $triple = makeMultiplier(3); and echo $triple(5) . "\n";.
Example solution:
<?php
// Write makeMultiplier and call it via $triple
function makeMultiplier(float $factor) {
return fn($n) => $n * $factor;
}
$triple = makeMultiplier(3);
echo $triple(5) . "\n";Test yourself
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What is a higher-order function?
- A function that adds two numbers
- A function that takes or returns a function
- A function that only prints text
- A method inside a class
Show answer
A function that takes or returns a function — Correct. A higher-order function takes a function as a parameter or returns one.
-
Match the higher-order patterns
- makeMultiplier(2)
- Returns a closure
- array_map(...)
- Accepts a closure
- callable parameter
- Accepts a function
-
A function that returns a function
<?php function makeMultiplier($factor) { return ____ ($n) => $n * $factor; } $double = makeMultiplier(2); echo $double(5) . "\n";Show answer
<?php function makeMultiplier($factor) { return fn ($n) => $n * $factor; } $double = makeMultiplier(2); echo $double(5) . "\n";