a simple divide-and-conquer, return-the-index-of-a-value-if-it-exists-in-an-array function :)

bisect.php

<?php 

function bisect($haystack, $needle, $trace = false)
{
    $min   = 0;
    $max   = count($haystack);
    $match = false;

    do {

        $mid = (int) (($max + $min) / 2);
        $val = $haystack[$mid];
        
        $loop = !(($min == $mid) || ($max == $mid));

        if ($needle == $val) {
            $match = true;
            break;
        } 
        
        ($needle > $val) ? $min = $mid : $max = $mid;

    } while ($loop);

    if ($trace) {
        printf("needle: %s; index: %s # => %s : ", $needle, $mid, $haystack[$mid]);
    }

    return $match ? $mid : false; 
}

// run!

system('clear');

$arr = range(0, 50, 5);

print_r($arr);

var_dump(bisect($arr, 101));
var_dump(bisect($arr, 51));
var_dump(bisect($arr, 50));
var_dump(bisect($arr, 49));
var_dump(bisect($arr, 31));
var_dump(bisect($arr, 1));
var_dump(bisect($arr, 0));
var_dump(bisect($arr, -1));

echo 'done';