<?php

$arr = [7,5, 6,2,4,1,0, 12, 15, 14, 18, 20, 21, 19];

sort($arr);

$min = null;
$max = null;

$count = count($arr);

for($index = 0; $index < $count; $index++) {
    $isLastIteration = ($count - $index === 1);

    if (!$isLastIteration and ($arr[$index] === ($arr[$index + 1] - 1))) {
        if ($min === null) {
             $min = $arr[$index];
        }

        continue;
    }

    if ($min !== null) {
        $max = $arr[$index];

        echo "$min -> $max", '<br>';

        $min = null;
        $max = null;
    }
}