Simple "fuzzy" dateInterval string creation

DateTimeUtil.php

<?php

namespace Tc\Std;

// Define these as descendands of \Exception or whaever
use Tc\Std\Exception\DateTimeInvalidArgument;
use Tc\Std\Exception\DateTimeUnexpectedResult;

abstract class DateTimeUtil
{

    public static function getApproximateDifferenceString(\DateTimeInterval $interval, $taxonomies = 2, $largest = 'y', $smallest = 'i')
    {
        $word = [];
        $convert = [
            'y' => 'year/s', 'm' => 'month/s', 'd' => 'day/s', 'h' => 'hour/s', 'i' => 'minute/s', 's' => 'second/s'
        ];

        $keys = array_keys($convert);
        $largestPos = array_search($largest, $keys, true);
        if($largestPos === null) {
            throw new DateTimeInvalidArgument("Invalid argument largest value '$largest'.  (Valid: " . implode(', ', $keys) . ")");
        }
        $smallestPos = array_search($smallest, $keys, true);
        if($smallestPos === null) {
            throw new DateTimeInvalidArgument("Invalid argument smallest value '$largest'.  (Valid: " . implode(', ', $keys) . ")");
        }

        foreach ($interval as $q => $v) {
            if ($v > 0) {
                $suffix = $convert[$q];
                $position = array_search($q, $keys, true);
                if($position === null) {
                    throw new DateTimeUnexpectedResult("Unexpected identifier '$q' from interval diff.");
                }

                if ($position >= $largestPos && $position <= $smallestPos) {
                    list($suffixSingle, $suffixPlural) = explode('/', $suffix, 2);
                    $part = "$v $suffixSingle";
                    if ($v > 1) {
                        $part .= $suffixPlural;
                    }
                    $word[] = $part;
                    if (count($word) >= 2) break;
                }
            }
        }
        return implode($word, ', ');
    }
 }