Factorial Calculator (Recursive Function)

<?php

// Thanks to Ben Doherty (@DOMSurfer) for helping me wrap my head around recursion!
function factorial($n) {
    return $n > 1 ? $n * factorial($n - 1) : 1;
}

/*
* My original version was much less elegant:
*
* function factorial($n) {
*   if( $n > 1 ) {
*       $n = $n * factorial( $n - 1 );
*   }
*   return $n;
* }
*/

echo '
    <form method="post" action="'.$_SERVER['PHP_SELF'].'">
        <label for="number">Number: </label>
        <input type="text" id="number" name="number" value="' . $_POST['number'] . '" autofocus>
        <input type="submit" name="submit" value="Get Factorial">
    </form>
';
if( $_POST['number'] ) { echo '<p>Factorial: ' . factorial( $_POST['number'] ) . '</p>'; }

?>