生成数组值列表时奇怪的PHP行为

Question

I have a multidimensional PHP array of the following form:

Array
(
    [0] => Array
        (
            [id] => 45
            [date] => 2013-05-16
        )

    [1] => Array
        (
            [id] => 30
            [date] => 2013-12-10
        )

    [2] => Array
        (
            [id] => 26
            [date] => 2014-03-27
        )

    [3] => Array
        (
            [id] => 34
            [date] => 2014-03-27
        )

)

I am trying to generate a list of the [id] values, separated by commas, using the following PHP code:

foreach ($my_array as $key => $value) { 
    if ($key == 0) {
        $id_list = $value[id];
    }
    if ($key !== 0 ) {
        $id_list .= "," . $value[id];
    }
}

I was hoping this would return

45,30,26,34

...but for some reason it returns

45,30,26,26

i.e. the penultimate ID is duplicated and the final ID is missed off. I have been staring at this for a while now but I can't see where I'm going wrong. Have I missed something obvious?

Answer 1

The better solution would be to not use those if() at all:

$ids = array();
foreach($arr as $val) {
   $ids[] = $val['id'];
}

$id_str = implode(',', $ids);