Recursive Functions

In this lesson you will learn how to write a recursive function in PHP and understand the basic idea of a function that calls itself. This is for anyone searching for "PHP recursive function how to use".

A recursive function is a function that calls itself. It suits work that repeats the same calculation pattern, such as "n! = n x (n-1)!". Always provide a stopping condition, or the function will call itself forever and the program will crash.

The sample code has factorial keep calling itself in the form $n * factorial($n - 1) until it reaches the stopping condition $n <= 1. Writing out the steps of computing the factorial of 5 by hand really helps it click.

A common stumbling block for beginners is forgetting that stopping condition and recursing forever. Deep recursion also consumes a lot of stack, so it is worth considering whether a loop would do instead. How deep is safe varies by environment.

In real-world development, recursion is a natural fit for hierarchical data such as folder trees or nested comment replies. Understand the mechanism once and you will find it applies widely.

📖 Reference code

<?php
function factorial($n) {
    if ($n <= 1) return 1;
    return $n * factorial($n - 1);
}
echo factorial(5);

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run PHP directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).