PHP - verify data types

verifyData.php

<?php

function verifyData (array $fields, array $data, array $excludeFields = []) {
  $array = [
    'data'  => [],
    'debug' => []
  ];

  foreach ($fields as $key => $value) {
    // If key is in exclude: ignore field
    if (!empty($excludeFields) && in_array($key, $excludeFields)) {
      continue;
    }

    $type = gettype($data[$key]);

    // How can I make null count as a string?
    // If data type is null, and it's field value is a string it should NOT get added to $array['data']
    if ($type !== $value || ($value === 'string' && is_null($data[$key]))) {
      $array['data'][] = [
        'field'   => $key,
        'message' => "Type of '$key' field is incorrect. Current type is: '$type', it should be: '$value'"
      ];
    } else {
      $array['debug'][] = "$key, $type, $value";
    }
  }

  print_r($array);
  echo '<hr>';

  // return $array;
}



// -----------------------------------------------------------------------------
// Tests
// -----------------------------------------------------------------------------
echo '<pre>';

// -----------------------------------------------------------------------------

$fields  = ['id' => 'integer', 'contents' => 'string', 'display' => 'boolean'];
$data    = ['id' => 123,       'contents' => 'hello',  'display' => true];
$exclude = [];

echo 'Output OK <br>';
verifyData($fields, $data, $exclude);

// -----------------------------------------------------------------------------

$fields  = ['id' => 'integer', 'contents' => 'string', 'display' => 'boolean'];
$data    = ['id' => 123,       'contents' => 'hi',     'display' => true];
$exclude = ['id'];

echo 'Output OK - Field "id" is excluded from debug output <br>';
verifyData($fields, $data, $exclude);

// -----------------------------------------------------------------------------

$fields  = ['id' => 'integer', 'contents' => 'string', 'display' => 'boolean'];
$data    = ['id' => 123,       'contents' => 123,      'display' => true];
$exclude = [];

echo 'Output OK - Field "contents" should not be an integer <br>';
verifyData($fields, $data, $exclude);

// -----------------------------------------------------------------------------

$fields  = ['id' => 'integer', 'contents' => 'string', 'display' => 'boolean'];
$data    = ['id' => 123,       'contents' => null,     'display' => true];
$exclude = [];

echo 'Output failed - Field "contents" should be in the debug output (null should be counted as a string) <br>';
verifyData($fields, $data, $exclude);