<?php

function convertFileSize($sizeInBytes, $unit = '')
{
    $units = ['B', 'KB', 'MB', 'GB', 'TB'];
    $index = 0;
    while ($sizeInBytes >= 1024 && $index < count($units) - 1) {
        $sizeInBytes /= 1024;
        $index++;
    }
    if ($unit && in_array($unit, $units)) {
        return round($sizeInBytes, 2) . ' ' . $unit;
    } else {
        return round($sizeInBytes, 2) . ' ' . $units[$index];
    }
}

// Example usage:
$fileSizeBytes = 1024 * 1024 * 5; // 5 MB
echo convertFileSize($fileSizeBytes) . "\n"; // Output: 5 MB

// Convert to a specific unit
echo convertFileSize($fileSizeBytes, 'KB') . "\n"; // Output: 5120 KB

?>