[7.x] Optimize Arr::set() method

[nguyenyou]

Same idea as #32192

Replace while:array_shift with foreach:unset. This operation is 3.5x faster. It is arguably more readable as well.

In my isolation tests with 1000 iterations:

• While loop 1.969ms
• Foreach loop 0.527ms

I made a small performance test and the result really surprised me. It is a significant performance difference that made me think there's something wrong with my test code.

<?php

function testData() {
  return array_fill(0, 100000, 'TEST DATA');
}

$array = testData();
$start = microtime(true);
$storage = [];
while(count($array) > 1) {
  $element = array_shift($array);
  $storage[] = $element;
}
$whileLoopTime = round(microtime(true) - $start, 4);
$elements = count($storage);
echo "whileloop: {$whileLoopTime} | storage stores {$elements} elements\n";

$array = testData();
$start = microtime(true);
$storage = [];
foreach($array as $i => $element) {
  if(count($array) === 1) {
    break;
  }
  unset($array[$i]);
  $storage[] = $element;
}
$foreachTime = round(microtime(true) - $start, 4);
$elements = count($storage);
echo "foreach: {$foreachTime} | storage stores {$elements} elements\n";

?>

---

• 1st run

whileloop: 8.0251 | storage stores 99999 elements
foreach: 0.0145 | storage stores 99999 elements

• 2nd run

whileloop: 7.9512 | storage stores 99999 elements
foreach: 0.0154 | storage stores 99999 elements

• 3rd run

whileloop: 7.9988 | storage stores 99999 elements
foreach: 0.0132 | storage stores 99999 elements

[browner12]

I'm glad we're seeing these performance improvements, but it got me curious as to why there is such a significant difference.

Turns out array_shift is a time consuming process when you have very large arrays, because every time it is run, it needs to completely re-index the array.