var_dump changes the result of the function?

I'm on a script which is going to make automatic appointment and the snippet below is for finding an available time. However, I've come across something that i've never h开发者_如何学运维eard of before.

When i use var_dump in my script to see the variables(i always do that when coding) the scripts run OK. However, when i remove these lines, script stops working as intended. as i cannot use var_dump for debugging since it makes the script running.

function getfirstfree($length, $limit) {
    $length = new DateInterval('PT' . $length . 'M');
    $table = "randevu";
    $today = date('Y-m-d', time());
    $q = $this->db->get_where('randevu', array('date' => $today));
    $events = $q->result_array();
    if ($q->num_rows() > 0) {
        for ($i=0; $i < sizeof($q); $i += 1) { 
            $c_ends = new DateTime($events[$i]['ends']);
            $n_starts = new DateTime($events[$i+1]['starts']);
            $freetime = $c_ends->diff($n_starts);
            $length_w_break = $length;
            $length_w_break->i = $length_w_break->i + 10;
            var_dump($length_w_break);
            var_dump($freetime);
            if ($freetime > $length_w_break) {
                echo "ok";
                return $c_ends->add(new DateInterval('PT10M'));
            }
            else {
                return "no";
            }
        }
    }

}

without var_dump the scripts returns no. with var_dump, it returns the first free slot.

current values taken from the database is

1-> starts 12:56:09 ends 12:56:09

2-> starts 13:33:11 ends 13:33:11

update var_dump($freetime); changes the output

In current PHP versions, DateInverval object and calculations with DateTime objects cause this bug. Avoid using them and use old-style, calculation with unix-time methods.