<?php
declare(strict_types=1);

function calculate_duration_hours(?string $start, ?string $end, ?float $manualHours = null): ?float
{
    if ($manualHours !== null) {
        return $manualHours > 0 ? round($manualHours, 2) : null;
    }
    if ($start === null || $end === null || !preg_match('/^\d{2}:\d{2}$/', $start) || !preg_match('/^\d{2}:\d{2}$/', $end)) {
        return null;
    }
    [$startHour, $startMinute] = array_map('intval', explode(':', $start));
    [$endHour, $endMinute] = array_map('intval', explode(':', $end));
    if ($startHour > 23 || $endHour > 23 || $startMinute > 59 || $endMinute > 59) return null;
    $minutes = ($endHour * 60 + $endMinute) - ($startHour * 60 + $startMinute);
    return $minutes > 0 ? round($minutes / 60, 2) : null;
}