Get uptime, memory usage, cpu usage and disk usage on GNU/Linux server using PHP

stats.php

<?php

header('Content-Type: application/json');

function get_uptime()
{
    $str   = @file_get_contents('/proc/uptime');
    $num   = floatval($str);
    $secs  = (int) fmod($num, 60); $num = intdiv($num, 60);
    $mins  = $num % 60;      $num = intdiv($num, 60);
    $hours = $num % 24;      $num = intdiv($num, 24);
    $days  = $num;

    return "$days day $hours hour $mins min $secs sec";
}

function get_server_memory_usage()
{
    $free = shell_exec('free -mh');
    $free = (string)trim($free);
    $free_arr = explode("\n", $free);
    $mem = explode(" ", $free_arr[1]);
    $mem = array_filter($mem);
    $mem = array_merge($mem);

    return $mem;
}

function get_server_cpu_usage()
{
   return trim(shell_exec("grep 'cpu ' /proc/stat | awk '{usage=($2+$4)*100/($2+$4+$5)} END {print usage \"%\"}'"));
}

function format_b($bytes)
{
    return round(($bytes / (1024 * 1024 * 1024)), 2).'GB';
}

$memory = get_server_memory_usage();

$data = [
    'uptime' => get_uptime(),
    'memory_total' => $memory[1],
    'memory_used' => $memory[2],
    'memory_free' => $memory[3],
    'cpu' => get_server_cpu_usage(),
    'disk_total' => format_b(disk_total_space('/')),
    'disk_free' => format_b(disk_free_space('/')),
];

echo json_encode($data);