Using PHP and oci8 to handle a query - php

Question

This is a past paper question for a database class that I'm stuck with. I'm just preparing for my exam, so it's okay to give away the answer.
Consider the following schema:
Borrow(userid: string, callnum: string, copynum: integer, checkout: date, return: date)
Here is the PHP function that has an error.
function countCheckedOutBookCopies($callnum){
$sql = "SELECT COUNT(*) AS bookcount FROM borrow
WHERE return = null and callnum = '".$callnum."'";
$stid = oci_parse($this->conn, $sql); //assume $this->con is correct
if($row = oci_fetch_object($stid)){
return $row->bookcount;
} else{
return -1;
}
}
There are 3 questions.
1.Find the error and fix it.
2.Another error occurs, fix it.
3.Despite everything being fixed, the function would return -1 all the time. Why is this?
I'm only familiar with procedural PHP using MySQL. But I tried running the code and was resulted with $stid returning boolean all the time because I don't know which part is right and which part is wrong.
Here are the things I've tried
1.Changing '".$callnum."' to just simply '$callnum' (because this is how I've always done it in MySQL)
2.Changing return = null to return = 'null' (but I don't think this is the case)
3.Maybe there is something wrong with the concept of getting COUNT(*) instead of just *
EDIT: Just a thought: I feel like oci8 and MySQL do pretty much the same thing, but is there a reason to prefer one over another? I'm sure MySQL is the more popular one, but my school seems to prefer using oci8 for exam questions
Thanks in advance!

Answer 1

Despite the "Find the error" and "Another error occurs":
1.) "return is null",
2.) this->conn is probably supposed to be this->con,
3.) the code is missing oci_execute($stid);,
4.) oci_free_statement($stid);.
function countCheckedOutBookCopies($callnum){
$sql = "SELECT COUNT(*) AS bookcount FROM borrow
WHERE return is null and callnum = '$callnum'";
$stid = oci_parse($this->con, $sql); //assume $this->con is correct
oci_execute($stid);
$ret = -1;
if($row = oci_fetch_object($stid)){
$ret = $row->bookcount;
}
oci_free_statement($stid);
return $ret;
}