Code loops optimization in PHP - php

Question

Is there any way to optimize this piece of code to work faster? I'd appreciate any suggestions!
This piece of code processes the transferring of edges during graph creation.
foreach($times_arrival as $city_id => $time_points) {
// if city is not prohibited for transfers and there is and exists any departure times for this city
if (isset($times_departure[$city_id]) && isset($cities[$city_id]))
{
foreach($times_arrival[$city_id] as $t1_info)
{
foreach($times_departure[$city_id] as $t2_info)
{
if ($t1_info[0] != $t2_info[0]) //transfers are allowed only for different passages
{
$t1 = $t1_info[1];
$t2 = $t2_info[1];
$vertex_key = new Vertex($city_id, $t1, 1);
$vertex_key = $vertex_key->toString();
//minimum transfer time is 10 min.
if (date('H:i', strtotime($t2)) > date('H:i', strtotime('+ 10 minutes', strtotime($t1))))
{
$this->graph[$vertex_key][] = new Edge(
NULL,
$vertex_key,
new Vertex($city_id, $t2, 0),
(float) 0,
$f((strtotime($t2) - strtotime($t1)) / 60, 0, 1) //edge weight
);
}
//if transfer is on the bound of the twenty-four hours
else if (date('H:i', strtotime('+ 24 hours', strtotime($t2))) > date('H:i', strtotime('+ 10 minutes', strtotime($t1))))
{
$this->graph[$vertex_key][] = new Edge(
NULL,
$vertex_key,
new Vertex($city_id, $t2, 0),
(float) 0,
$f(strtotime('+ 24 hours', strtotime($t2)) - strtotime($t1) / 60, 0, 1) //edge weight
);
}
}
}
}
}
}
example of variables:
var_dump($times_arrival); //$times_departure have the same structure
array
3 =>
array
0 =>
array
0 => string '1' (length=1)
1 => string '08:12' (length=5)
1 =>
array
0 => string '2' (length=1)
1 => string '08:40' (length=5)
41 =>
array
0 =>
array
0 => string '21' (length=2)
1 => string '12:40' (length=5)

Answer 1

Thank you all!
The reason of slow speed was coz of using functions strtotime() and date().

Answer 2

In that case only you can say whether you chose a good or bad algorithm. In my point of view your code not has no extra computations.
Only one recommendation - use Xdebug to profile your code and find out where the bottleneck is, if possible.