Merge random array 2 (letters) into ordered array 1 (numbers)

merge.php

<?php

$final = array();
$left = array(0,1,2,3,4,5,6,7,8,9,10,11,12,13,14);
$right = array('a','b','c','d','e');
shuffle($right);

$merged = array_merge($left, $right);
shuffle($merged);
$r = 0;

foreach($merged as $key => $value) {

  if(in_array($value, $right)) {
    $r++;
  }

  // numbers
  if(in_array($value, $left) && $key-$r >= 0) {
      $final[$key] = $left[$key-$r];
  }

  // letters
  if(in_array($value, $right)) {
    $final[$key] = $value;
  }

}

foreach($final as $key => $value) {
  echo 'Key: '.$key.' Value: '.$value.'<br />';
}

?>