Remove entry with specific attributes in php arrays

I have the following array:

$entries = Array
(
    (29) => Array
        (
            (name) => Markus Florida
            (email-adresse) => mail@example.com
            (created) => 2021-04-05 21:38:06
            (form_id) => cf_34
            (time) => vor 03:08h
        )

    (30) => Array
        (
            (name) => Joe Doe
            (email-adresse) => joe@example.com
            (created) => 2021-04-05 21:38:06
            (form_id) => cf_34
            (time) => vor 03:08h
        )

    (31) => Array
        (
            (name) => Max Muster
            (email-adresse) => max@example.com
            (created) => 2021-04-05 21:38:06
            (form_id) => cf_34
            (time) => vor 03:08h
        )

    (32) => Array
        (
            (name) => Jorlina Doe
            (email-adresse) => jorlina@example.com
            (created) => 2021-04-05 21:38:06
            (form_id) => cf_38
            (time) => vor 03:08h
        )

)

The key in this array is the id of the entry.

I want to unset (delete) all entries whose “form_id” is not “cf_34”.

I tried it with this code, but it didn’t work:

  foreach ($entries as $entry) {
    if ( $entry("form_id") != "cf_34" ) {
      unset($entries($entry));
    }
  };

How can I get the current key in this array to delete the entry with this key?