PHP - Get Server Memory Usage

/* Gets server memory usage, can be used to debug how much memory a script is using */
function get_server_memory_usage(){
    $free = shell_exec('free -m');
    $free = (string)trim($free);
    $free_arr = explode("\n", $free);
    $mem = explode(" ", $free_arr[1]);
    $mem = array_filter($mem);
    $mem = array_merge($mem);
    $memory_usage = "used: " . $mem[2] . "\n";
    $memory_usage .= "free: " . $mem[2] . "\n";
    return $memory_usage;
}