Description:
------------
After updating from PHP 7.0.* to 7.2.5 next code start failing with

PHP Warning:  ExcelSheet::writeRow(): Failed to write cell in row 0, column 0 with error 'ok' in /home/ddc/young/htdocs/2.php on line 18


It use https://github.com/iliaal/php_excel module and you may think that the problem with this module, but this not true.

If i will comment array_walk, which in this case NOT changing data - it will work fine

With array_walk it gives error

but data array is the same with and without array_walk call

so array_walk call changing not only the data but something invisible in addition, but should not
 

Test script:
---------------
<?php

$data = array (
	0 => 'test me hard'
);


$xlBook = new \ExcelBook(null, null, true);
$xlBook->setLocale('UTF-8');
$xlSheet1 = $xlBook->addSheet('Sheet1');

var_dump($data);
array_walk($data, function(&$field) {
		$field = (string)$field;
	});
var_dump($data);

$xlSheet1->writeRow(0, $data);


Expected result:
----------------
Same result with and without array_walk

Actual result:
--------------
Error with array_walk and success without