Dealing with dynamic amounts of fields in HTML forms usually means you get your data in a very foreach-unfriendly manner. This helper function is meant to address that. Input and output are presented in JSON because I like it better. Actual results are PHP. YMMV

arrayFromHTMLInput.php

<?php

/**
 * Helper function to reformat arrays.
 * @see https://gist.github.com/Sleavely/2c1aa7ba13da80dfeb43
 * 
 * @param array $source
 *   The multidimensional array where each input field is an array
 * @param array $arrays
 *   A list of which keys to include in the output
 * @return array
 *   An array where each entry represents an object (e.g. an attendant).
 */
function arrayFromHTMLInput($source, $arrays)
{

  $output = array();
  foreach($arrays as $field)
  {
    if(isset($source[$field]) && is_array($source[$field]))
    {
      foreach($source[$field] as $i => $value)
      {
        if(!isset($output[$i]))
        {
          $output[$i] = array();
        }
        $output[$i][$field] = $value;
      }
    }
  }
  
  return $output;
}

example.php

<?php

$input = json_decode(file_get_contents('input.json'));
require_once('arrayFromHTMLInput.php');

// This will produce the same kind of output as output.json
$output = arrayFromHTMLInput(
  $input,
  array('firstname', 'lastname', 'email', 'phone')
);

input.json

{
  "firstname":[
    "John",
    "Eric",
    "Alex"
  ],
  "lastname":[
    "Johnson",
    "Ericsson",
    "Andersen"
  ],
  "email":[
    "john@example.com",
    "eric@example.com",
    "alex@example.com"
  ],
  "phone":[
    "555-123 123 1",
    "555-123 123 2",
    "555-123 123 3"
  ],
  "someRandomData":"XYZ",
  "moreRandomData":1337
}

output.json

[
  {
    "firstname":"John",
    "lastname":"Johnson",
    "email":"john@example.com",
    "phone":"555-123 123 1"
  },
  {
    "firstname":"Eric",
    "lastname":"Ericsson",
    "email":"eric@example.com",
    "phone":"555-123 123 2"
  },
  {
    "firstname":"Alex",
    "lastname":"Andersen",
    "email":"alex@example.com",
    "phone":"555-123 123 3"
  }
]