How to properly loop through array keys in PHP

Question

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?

Answer 1

Warning: the benchmark below is not accurate that makes its results wrong.

From my quick and dirty benchmark I did the following 50 times:

Solution 1 (worst):

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

Size of array: 1000000

• Min: 0.11363291740417
• Avg: 0.11681462287903
• Max: 0.14569497108459

Size of array: 9999999

• Min: 1.3098199367523
• Avg: 1.3250577354431
• Max: 1.3673560619354

---

Solution 2 (middle):

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

Size of array: 1000000

• Min: 0.10167503356934
• Avg: 0.10356130123138
• Max: 0.11027193069458

Size of array: 9999999

• Min: 1.2077870368958
• Avg: 1.2256314325333
• Max: 1.2829539775848

Solution 3 (best):

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

Size of array: 1000000

• Min: 0.090911865234375
• Avg: 0.092938437461853
• Max: 0.097810983657837

Size of array: 9999999

• Min: 1.0793349742889
• Avg: 1.0941110134125
• Max: 1.1402878761292

---

So you can see solution 3 is the quickest option if you only want to look through array keys :)

Hope this helps.

---

Code:

<?php
class DirtyBenchmarker {
  private $results = [];
  private $size_of_array;

  public function __construct($size_of_array)
  {
    $this->size_of_array = $size_of_array;
    echo 'Size of array: ' .  $this->size_of_array . PHP_EOL;
  }
  
  private function solution1() {
    $array = range(0, $this->size_of_array - 1);
    ob_start();
    $start = microtime(true);
    foreach (array_keys($array) as $array_key) {
      echo $array_key;
    }
    $finish = microtime(true) - $start;
    $echod = ob_get_clean();
    $this->results['solution1'][] = $finish;
  }

  private function solution2() {
    $array = range(0, $this->size_of_array - 1);
    ob_start();
    $start = microtime(true);
    foreach ($array as $array_key => $array_value) {
      echo $array_key;
    }
    $finish = microtime(true) - $start;
    $echod = ob_get_clean();
    $this->results['solution2'][] = $finish;
  }

  private function solution3() {
    $array = range(0, $this->size_of_array - 1);
    $array_keys = array_keys($array);
    ob_start();
    $start = microtime(true);
    foreach ($array_keys as $array_key) {
      echo $array_key;
    }
    $finish = microtime(true) - $start;
    $echod = ob_get_clean();
    $this->results['solution3'][] = $finish;
  }

  public function benchmark() {
    $this->solution1();
    $this->solution2();
    $this->solution3();
  }

  public function getResults()
  {
    echo PHP_EOL . 'Solution 1:' . PHP_EOL;
    echo 'Min: ' . min($this->results['solution1']) . PHP_EOL;
    echo 'Avg: ' . array_sum($this->results['solution1']) / count($this->results['solution1']) . PHP_EOL;
    echo 'Max: ' . max($this->results['solution1']) . PHP_EOL;

    echo PHP_EOL . 'Solution 2:' . PHP_EOL;
    echo 'Min: ' . min($this->results['solution2']) . PHP_EOL;
    echo 'Avg: ' . array_sum($this->results['solution2']) / count($this->results['solution2']) . PHP_EOL;
    echo 'Max: ' . max($this->results['solution2']) . PHP_EOL;

    echo PHP_EOL . 'Solution 3:' . PHP_EOL;
    echo 'Min: ' . min($this->results['solution3']) . PHP_EOL;
    echo 'Avg: ' . array_sum($this->results['solution3']) / count($this->results['solution3']) . PHP_EOL;
    echo 'Max: ' . max($this->results['solution3']) . PHP_EOL;
  }
}

$benchmarker = new DirtyBenchmarker(1000000);
$runs = 50;
for ($i = 0; $i < $runs; $i++) {
  $benchmarker->benchmark();
}
$benchmarker->getResults();

$benchmarker = new DirtyBenchmarker(9999999);
$runs = 50;
for ($i = 0; $i < $runs; $i++) {
  $benchmarker->benchmark();
}
$benchmarker->getResults();