find the nearest key, value, and delta from provided array of data.

nearest.php

<?php
/**
 * Example of usage and results
 * $a = [ 7, -10, 13, 8, 4, -7.2, -12, -3.7, 3.5, -9.6, 6.5, -1.7, -6.2, 7 ];
 * print_r(nearest(-3.1, $a)); // idx = 7, delta = 0.6, val = -3.7
 * print_r(nearest(7, $a)); // idx = 0, delta = 0, val = 7
 * print_r(nearest(0, $a)); // idx = 11, delta = 1.7, val = -1.7
*/
function nearest($needle, $haystack) {
    // do some basic check of inputs
    if (!is_array($haystack) || empty($haystack) || !is_numeric($needle)) {
        return false;
    }

    // I feel lucky
    $found = array_search($needle, $haystack, false);
    if ($found !== false) {
        return [
            'idx' => $found,
            'delta' => 0,
            'value' => $needle
        ];
    }

    // digging deeper by binary search
    asort($haystack); // sort values first

    // check edge case: lover than the most low value
    $firstKey = key($haystack);
    if ($haystack[$firstKey] > $needle) {
        return [
            'idx' => $firstKey,
            'delta' => abs($haystack[$firstKey] - $needle),
            'value' => $haystack[$firstKey]
        ];
    }
    // check edge case: bigger than the most biggest value
    $lastKey = key(array_slice($haystack, -1, 1, true)); //  array_key_last($haystack); > php7.2
    if ($haystack[$lastKey] < $needle) {
        return [
            'idx' => $lastKey,
            'delta' => abs($needle - $haystack[$lastKey]),
            'value' => $haystack[$lastKey]
        ];
    }

    // digging trough array
    $orderedKeys = array_keys($haystack);
    $start = 0;
    $end = count($haystack) - 1;
    $nearest = ['key' => null, 'delta' => null];
    $mid = null;

    while ($start <= $end) {
        // find middle index
        $mid = (int)floor(($start + $end) / 2);
        if ($needle < $haystack[$orderedKeys[$mid]]) {
            // move to the left side of the array
            $end = $mid -1;
        }
        else {
            // move to the right side of the array
            $start = $mid + 1;
        }

        $delta = abs($haystack[$orderedKeys[$mid]] - $needle);
        if (!isset($nearest['delta']) || $nearest['delta'] > $delta) {
            $nearest['key'] = $mid;
            $nearest['delta'] = $delta;
        }
    }

    return [
        'idx' => $orderedKeys[$nearest['key']],
        'delta' => $nearest['delta'],
        'val' => $haystack[$orderedKeys[$nearest['key']]]
    ];
}