Low level PHP making barcodes

Efficient barcodes

<?php

if ( !isset( $_GET['batch'] ) ) 
  die("YOU MUST SPECIFY A BATCH PARAMETER");

$batch = $_GET['batch'];

$dbc = @mysqli_connect( DB_HOST, DB_USER, DB_PASSWORD, DB_NAME ) or die();

$batch_size = 100000;

function numbers()
{
  return str_pad ( mt_rand( 0, 9999 ), 4, "0", STR_PAD_LEFT );
}

function letters()
{
  $charset = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
  $count = strlen($charset);
  $length = 3;
  $str = '';
  
  while ($length--) {
    $str .= $charset[mt_rand(0, $count-1)];
  }
  
  return $str;  
}

function get_random_code($batch)
{
  return $batch . letters() . numbers();
}

$start = time();

$q = "INSERT IGNORE INTO `barcodes` (`code`, `batch`) VALUES (?,?)";
$stmt = mysqli_prepare( $dbc, $q );
mysqli_stmt_bind_param( $stmt, 'ss', $code, $batch );

$i = 0;
while( $i < $batch_size )
{
  $code = get_random_code($batch);
  mysqli_stmt_execute( $stmt );
  $i += mysqli_stmt_affected_rows( $stmt );
}

$elapsed = time() - $start;
echo "$elapsed seconds";