Human age to dog age converter

human_to_dog.php

<?php

function human_to_dog_years($human_age) {

    if (!$human_age || $human_age < 0) {
        throw new InvalidArgumentException('Only positive input is accepted!');
    }
    
    $firstTwoHumanYear = 10.5;                  // 10.5 dog years
    $oneHumanYear = $firstTwoHumanYear / 2;     // 5.25 dog years
    $dogAge = 0;
    
    if ($human_age < 2) {
        $dogAge = $human_age * $oneHumanYear;
    } else {
        $dogAge = $firstTwoHumanYear + ($human_age - 2) * 4;
    }

    return $dogAge;

}

if (count($argv) < 2) {
    exit(sprintf("Usage: %s <age in human year>", $argv[0]));
}

$human_age = $argv[1];
$dog_age = human_to_dog_years($human_age);

echo sprintf("%s human year(s) in dog years: %s", $human_age, $dog_age);