Find a number within an array in recursive mode

binary_search.php

<?php
/**
 * Find a number within an array in recursive mode
 */
function findNumber($a, $num, $start, $end) {
    $mid = floor(($end - $start) / 2) + $start;
    if ($num > $a[$end] || $num < $a[0] || $start > $end) {
        return "number does not exist";
    }
    if ($a[$mid] == $num) {
        return $mid;
    } elseif ($a[$mid] < $num) {
        return findNumber($a, $num, $mid+1, $end);
    } else {
        return findNumber($a, $num, $start, $mid-1);
    }
}

$a = [1, 2, 3, 4, 5, 6, 7, 8];
echo findNumber($a, 1, 0, count($a)-1);