$arr = array(1 => 10, 1 => 11);

What happens when the interpreter reaches this line of code? This is not a syntax error and it is completely valid. Very similar, but more interesting case is when we have an array of identical keys, where those identical keys are represented once as an integer and then as a string.

$arr = array(1 => 10, "1" => 11);

Now several questions arise. First of all, how many elements have this array? Two or one. This can be easily verified by checking what count() will return.

echo count($arr);

The correct answer is 1. This simply means, that there’s no difference between string keys and integer keys. What would happen if we had a “normal” array with different keys?

$arr = array(1 => 10, "2" => 11);
echo count($arr);

As expected this returns 2.

Next thing to check is what’s in the array after this initialization line.

$arr = array(1 => 10, "1" => 11);

Is there something in the first element $arr[0], or there’s something in the second element $arr[1]? What is the value of the single value?
As it appears the second element replaces the first one. We’ve seen that the array has only one value, but where’s that value? The only way to check this is to dump both elements:

var_dump($arr);

Here we can see that $arr[1] contains “11” and it is the only value, and $arr[0] is not set.

The Task

You’ve one array with tickets of linked destinations – so you start from one city to another, than next and so on. Each element is an array with “from” and “to” destinations:

$inputTickets = array(
    0 => array('from' => 'barcelona', 'to' => 'madrid'),
    1 => array('from' => 'sofia', 'to' => 'paris'),
    2 => array('from' => 'madrid', 'to' => 'milano'),
    3 => array('from' => 'paris', 'to' => 'barcelona'),
    4 => array('from' => 'cupertino', 'to' => 'sofia'),
    5 => array('from' => 'milano', 'to' => 'valencia'),
    6 => array('from' => 'valencia', 'to' => 'nice'),
    7 => array('from' => 'mountain view', 'to' => 'cupertino'),
);

It’s easy to construct two arrays – the “from” destinations array and the “to” destinations array, and here the easiest way to get the starting point of the whole trip, because of the fact that these are linked tickets.

$fromDestinations = $toDestinations = array();

foreach ($inputTickets as $k => $v) {
    $fromDestinations[] = $v['from'];
    $toDestinations[]   = $v['to'];
}

// and finally get the starting point
$startPoint = array_diff($fromDestinations, $toDestinations);

Conclusion

Beside of knowing several algorithm techniques, there’s also need of knowing the language syntax and possibilities – in that case PHP’s

Recently I wrote about list() in PHP which is indeed very powerful when assigning variable values from array elements.

$a = array(10, array('here', 'are', 'some', 'tests'));
list($count, $list) = $a;

Actually my example in the post was not correct, because I wrote that you can pass an associative array, but the truth is that you cannot, and thus the array should be always with numeric keys. After noticing the comments of that post, and thanks to @Philip,  I searched a bit about how this problem can be overcome.

There is a Solution

As always PHP gives a perfect solution! You can see on the list() doc page that there is a function that may help you use an associative array.

Extract

extract() is perhaps less known than list(), but it does the right thing!

$a = array('count' => 10, 'list' => array('here', 'are', 'some', 'tests'));
extract($a);

echo $count;    // 10
print_r($list); // array('here'....

Note that now both $count and $list are defined.