Warning: DOMDocument::save() [domdocument.save]: xmlEscapeEntities : char out of range

Question

I have while loop and which is has 20 iterations. In one iteration it make of 500 products in XML format. It works fine when the products are 200, but if I decrease the products then it need to 50 iterations, which gives time exceed error.

I don't know how to balance this. If I fix one error of time exceed then the above titled error occurs but I fix this then time exceed error.

I am passing my products XML to Volusion API.

My Code is following.

<?php 

// Developed BY Doulat Khan Technology Architects Islamabad. 

$conn= mysql_connect('localhost, 'username', 'password');
if (!$conn) {
    die('Could not connect: ' . mysql_error());
}

ini_set('max_execution_time', 300); //300 seconds = 5 minutes

$query = "SELECT * FROM `Vol_import_count` WHERE `imp_id` = '1'";
mysql_select_db('mydb');
$retval = mysql_query($query);
//print_r($retval);
if(! $retval )
{
  die('Could not enter data: ' . mysql_error());
}
while($row = mysql_fetch_assoc($retval))
{

  $cur_itration = $row['cur_itration'];
  $total_itration = $row['total_itration'];
        $status = $row['status'];
  $offSet = $row['offSet'];
    
}

$cur_itration = 1;
$offset = 0;
$malab_url ="https://online.malabs.com/mws/items/?limit=500&offset=$offset";
while($cur_itration <= $total_itration) {
 
$new_url = $malab_url."&offset=$offset";
$offset += 500;
   
$cur_itration++;    

include 'malab2vol.php';

include 'importPro.php';

include 'assignCat.php';

} 

?>