Вы используете mysqli_query в mysqli_query, и это не может работать. Посмотрите ниже, как это должно быть.
/**
* Build new customer number
* @return varchar custNo : Newly build customer number
*/
function buildCustNo(){
/*Establish Database connection
*/
$link = mysqli_connect("localhost","root","","mysaccodb") or die("Unable to Connect to Database");
// Determine biggest customer ID// Determine biggest customer ID
$sql_maxID = "SELECT MAX(cust_id) AS maxid FROM customer";
$query_maxID = mysqli_query($link, $sql_maxID) or die ("Error" . mysqli_error($link));
checkSQL($query_maxID);
$result_maxID = mysqli_fetch_array($query_maxID);
// Read customer number format
$cnParts = explode("%", $_SESSION['set_cno']);
$cnCount = count($cnParts);
// Build customer number
$i = 0;
$custNo = "";
for ($i = 1; $i < $cnCount; $i++) {
switch($cnParts[$i]){
case "N":
$custNo = $custNo.($result_maxID['maxid'] + 1);
break;
case "Y":
$custNo = $custNo.date("Y",time());
break;
case "M":
$custNo = $custNo.date("m",time());
break;
case "D":
$custNo = $custNo.date("d",time());
break;
default:
$custNo = $custNo.$cnParts[$i];
}
}
// Return customer number
return $custNo;
}