Refactor array_merge out of loops

[kingkero]

I have read the CONTRIBUTING.md, as this does not introduce a new feature and touches very few points in general, I did decide against opening an issue first. I hope that is ok.

array_merge() inside loops is quite slow. Take a look at the following PHPBench example:

<?php

class ArrayMergeBench
{
  /**
   * @Revs(10000)
   * @Iterations(5)
   */
  public function benchMergeInLoop()
  {
    $result = [];

    for ($i=0; $i<100; $i++) {
      $result = array_merge($result, [$i]);
    }
  }

  /**
   * @Revs(10000)
   * @Iterations(5)
   */
  public function benchMergeOutsideLoop()
  {
    $result = [];

    for ($i=0; $i<100; $i++) {
      $result[] = [$i];
    }

    $finalResult = array_merge(...$result);
  }
}

returns	benchmark	subject	set	revs	its	mem_peak	mode	rstdev
ArrayMergeBench	benchMergeInLoop		10000	5	2.808mb	23.545μs	±3.24%
ArrayMergeBench	benchMergeOutsideLoop		10000	5	2.808mb	6.114μs	±4.93%

So even with a single element in the array, merging outside the loop is already 4x faster. The more elements in the loop, the slower merging inside will be.

[sebastianfeldmann]

Valid point :)