A function that calls itself. Every recursive function needs:
Use recursion for problems naturally structured as smaller sub-problems (trees, nested data, math sequences).
<?php
// Factorial
function factorial(int $n): int {
if ($n <= 1) return 1; // base case
return $n * factorial($n - 1); // recursive case
}
echo factorial(5); // 120
// Fibonacci
function fib(int $n): int {
if ($n <= 1) return $n;
return fib($n - 1) + fib($n - 2);
}
// Flatten nested array
function flatten(array $arr): array {
$result = [];
foreach ($arr as $item) {
if (is_array($item)) {
$result = array_merge($result, flatten($item));
} else {
$result[] = $item;
}
}
return $result;
}
?>