Recursion in PHP

"In order to understand recursion, one must first understand recursion." - Anonymous

Today I am going to give an example of recursion in PHP. There are some cases where recursion will be a more elegant solution to a given problem than iteration will be.

If you don't already know what recursion is, the simplest way I can explain it is recursion is a function that calls itself.

To give you a better idea of how you can use recursion, here is a very simple example.

Let's say you want to be able to call a function that will count down from whatever number you send as an argument to zero ... but you don't want to use a loop. Instead you could do the following:

//recursive count down function 
function count_down($number) { 
    // check to make sure we have been sent a number
    if (is_numeric($number)) {
        return false;
    }

    // echo pause between numbers
    echo ' ... ';

    // if $number is less then 1, we end our count down
    if ($number < 1) { 
        // echo final output
        echo 0; 
    } else { 
        // if we can still continue our count down we echo the number to the 
        // screen and call count_down again, as you can see this could also be 
        // done in a loop, but for our purpose we are using a recursive function
        echo $number;
        count_down($number-1); 
    } 

    return true;
} 

// call to our count down function and see if it was successful
if (count_down(10)) {
    echo ' ... Lift Off!';
} else {
    echo 'You are doing it wrong!';
} 

This function would output the following:

... 10 ... 9 ... 8 ... 7 ... 6 ... 5 ... 4 ... 3 ... 2 ... 1 ... 0 ... Lift Off!

Obviously this is a very basic example and further study is recommended, but I hope that I have made the concept a little bit easier to grasp.