'How to properly loop through array keys in PHP [closed]

If I have an array with multiple keys and values, like this:

$array = array(
  'key1' => 'value1',
  'key2' => 'value2',
);

Are there any best-practices on how to loop through an array when I only use the keys?

Possible solution 1:

foreach(array_keys($array) as $array_key) {
  echo $array_key;
}

Possible solution 2:

foreach($array as $array_key => $array_value) {
  echo $array_key;
}

I can see the advantage in solution 1 as it doesn't include the unused variable $array_value. Is there a difference in performace (speed, memory allocation, etc.) between these two?

phparraysperformanceloopscoding-style


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