Example Recursive Function in PHP

A recursive function is defined simply as a function that calls itself. (Also falling under the definition would be any group of functions that loop among themselves, such as function A calling function B, which in turn calls A.) PHP supports recursive functions, and they are often an elegant solution instead of a lengthy, deep, set of static instructions. Let's look at a simple comparison of two functions that do the same thing, one written recursively, and the other not.

Recursive function example

<?php
function sum_up_to($number) {
    $result = 0;
    for ($i = 1; $i <= $number; $i++) {
        $result += $i;
    }
    return $result;
}

// The same function, written as a recursive function
function sum_up_recursive($number) {
    return ($number) ? ($number + sum_up_recursive($number - 1)) : 0;
}

// Try this in both cases, with the number 42, which should return: 903
echo 'Regular version = ', sum_up_to(42), "\n";
echo 'Recursive version = ', sum_up_recursive(42), "\n";
?>

Notice how even in this simple example of adding numbers, the code for the recursive version is much shorter and simpler. The only extra effort that must always go into recursive functions is ensuring that they exit. It is easy to write a function that will run forever. In this case, we are using the recursive function to count down, so we need to detect when we are at zero and return zero, ending the recursion.

When writing recursive functions, keep in mind two issues. The first issue is execution time.By default, PHP restricts the execution time of any script. This helps ensure that a script does not lock up a web server. Hence, you should make sure that any recursion will complete within the allotted time or modify the limit using set_time_limit().

The second issue is resources. With each call to a function, memory is allocated for all arguments passed by value and for all local variables created in a function. This memory is released on exit of that function call. Normally, this is not an issue because a function is called, does its job, and then exits. However, when a function is called recursively, this normal pattern is interrupted. When a function is called recursively, each call consumes whatever memory is needed until the recursion finally ends, when each recursive call starts returning. Normally this is not an issue. However, it is good practice to ensure that recursive functions do not create large arrays or use other "consumable" resources, such as opening files.

Finding how to end your recursion properly is the most important skill to learn in this regard. Hope it helps.