PHP array_diff_key() is empty but should not - php

Question

I query a series of tables in order to print these, and I want to print all keys of each table even if the values are empty, but a specific set of keys which are the same for each table shall not be printed.
My query and fetch of the result in an array for one table:
$stmt = $db_conn->prepare("SELECT * FROM table;");
$stmt->execute();
$array = $stmt->fetchAll(PDO::FETCH_ASSOC);
$stmt->closeCursor();
var_export($array);returns:
array ( 0 => array ( 'a' => '1', 'b' => '2018-12-21', 'c' => '', 'd' => '', ), )
I prepared a list of bad keys in an array:
var_export($bad_keys);returns:
array ( 0 => array ( 'a' => '1', 'b' => '2019-01-05', ), )
For each table I want to exclude the bad keys {a, b} from the query result in $array by use of array_diff_key():
$array_new = array_diff_key($array, $bad_keys);
var_dump($array_new); returns empty:
array(0) { }.
$array_new should have the keys {'c', 'd'} but it has not. I don't see a mistake in my code. Please help.

Answer 1

array_diff_key is used to compute the difference between the keys of two or more arrays, i.e. a new array with all key/value pairs of the first array argument having keys not existing in any other array argument's keys are returned.
In your code, you try to compare the keys of your first array with an array of values (i.e. your $bad_keys, actually having numbered indexes), not the keys of the $bad_keys array. This is not how array_diff_key works.
Be sure to check out the reference at: http://php.net/manual/en/function.array-diff-key.php
One method giving you only the keys you are looking for as an array:
Just create a new array of the keys of the first array as values and then use array_diff to compare it with your $bad_keys.
To get the key/value pairs you can use this one (as Quasimodo's Clone suggested):
array_diff_key($array, array_flip($bad_keys))
UPDATE: The keys in the first array are also not at the same level as the keys in the $bad_array.

Answer 2

My mistake: In array_diff_key() I compared two multidimensional arrays without writing the index [0] for each array to be compared.
Solution: I added the index [0] to each array to be compared, and additionally I created an array from the result of array_diff_key()
$array = array( 0 =>
array(
'a' => '',
'b' => '',
'c' => '',
'd' => '',
),
);
$bad_keys = array( 0 =>
array(
'a' => '',
'b' => '',
),
);
$array_new = array( array_diff_key( $array[0], $bad_keys[0] ));
var_export( $array_new );
now returns the desired result:
array( 0 =>
array(
'c' => '',
'd' => '',
),
);