Get first key in a (possibly) associative array?


What's the best way to determine the first key in a possibly associative array? My first thought it to just foreach the array and then immediately breaking it, like this:

foreach ($an_array as $key => $val) break;

Thus having $key contain the first key, but this seems inefficient. Does anyone have a better solution?

7/15/2013 3:02:33 AM

Accepted Answer

2019 Update

Starting from PHP 7.3, there is a new built in function called array_key_first() which will retrieve the first key from the given array without resetting the internal pointer. Check out the documentation for more info.

You can use reset and key:

$first_key = key($array);

It's essentially the same as your initial code, but with a little less overhead, and it's more obvious what is happening.

Just remember to call reset, or you may get any of the keys in the array. You can also use end instead of reset to get the last key.

If you wanted the key to get the first value, reset actually returns it:

$first_value = reset($array);

There is one special case to watch out for though (so check the length of the array first):

$arr1 = array(false);
$arr2 = array();
var_dump(reset($arr1) === reset($arr2)); // bool(true)
3/10/2019 2:00:53 AM

array_keys returns an array of keys. Take the first entry. Alternatively, you could call reset on the array, and subsequently key. The latter approach is probably slightly faster (Thoug I didn't test it), but it has the side effect of resetting the internal pointer.

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