numberOfCarryOperations.php

<?php

/*
Find the number of carry operations when sum two numbers. For example
the carry operations for 66+75 is 2 because:

11
 66
+75
–––
141

6+5 = 11, then have 1 and carry 1 (first carry operation)
6+7+1 = 14, then have 4 and carry 1 (second carry operation)
1+0 = 1, then have 1 and carry none
*/
function numberOfCarryOperations(int $a, int $b) : int
{
    $arr = array_reverse(array_map('intval', str_split((string) $a)));
    $brr = array_reverse(array_map('intval', str_split((string) $b)));
    $len = count($arr) > count($brr) ? count($arr) : count($brr);
    $crr = 0;
    $num = 0;

    for($i = 0; $i < $len; $i++) {
        $aDgt = isset($arr[$i]) ? $arr[$i] : 0;
        $bDgt = isset($brr[$i]) ? $brr[$i] : 0;

        $sum = $aDgt + $bDgt + $crr;

        if ($sum >= 10) {
            $strSum = (string) $sum;
            $num++;
            $crr = intval($strSum[0]);
        } else {
            $crr = 0;
        }
    }

    return $num;
}

$test = [
    [66, 75], // 2
    [123, 456], // 0
    [555, 555], // 3
    [900, 11], // 0
    [145, 55], // 2
    [0, 0], // 0
    [1, 99999], // 5
    [999045, 1055], // 5
    [101, 809], // 1
    [189, 209], // 1
];

foreach ($test as $params) {
    var_dump(numberOfCarryOperations(...$params));
}