MySQLI Count Error - php

Question

I have two different functions:
function user_count(){
global $db;
$query = mysqli_query($db,"SELECT COUNT(user_id) FROM users");
if($result = $query){
$user_count = mysqli_num_rows($result);
mysqli_free_result($result);
return $user_count;
}
}
In this function, I query the database to tell me how many users there are, and in PHP I use user_count(). The result of this function is 1. Which is correct, because I only have only 1 users in the database at the moment.
However when I use the same code for a different function such as:
function user_exists($username){
global $db;
sanitize($username);
$query = mysqli_query($db,"SELECT COUNT(user_id) FROM users WHERE username=".$username) or die ("Doesn't work");
if($result = $query){
$user_exists = mysqli_num_rows($result);
mysqli_free_result($result);
return $user_exists;
}
}
Not to mention I have also tried to comment out the sanitize($username). But even with it commented out I still am getting an error.
And I use the function user_exists("superadmin"), I get the error "Doesn't work". I am wondering what am I doing wrong in the second function where the first function would return the number of rows, where as second function would return an error.
I know for certain that:
Database is connecting
String sanitization is working
User does exists on the database
If would much appreciated if you reply using the procedural style.

Answer 1

To paraphrase a wise man, "This is not the count you are looking for"
mysqli_num_rows will return the number of rows in the result set which, in this case, will always be 1.
To get the actual count, you will have to use $row=mysqli_fetch_row($result) and then the count you want is in $row[0];
Also, you're not creating your query properly - the query should have single quote marks around the variable. Even better, look at using prepared statements as they will take care of all the escaping and quoting for you

Answer 2

You're asking for the number of user_ids in the table users, so the MySQL server will return something like this:
mysql> SELECT COUNT(user_id) FROM users;
+----------+
| count(*) |
+----------+
| 36 |
+----------+
1 row in set (0.00 sec)
Then, you're using mysqli_num_rows which checks the result of the query to see how many rows were returned. But, as you can see, there's "1 row in set" - so you're getting back the number of rows in the result of the query, not the number of user_ids.
What you want to do is something like this:
$row = mysqli_fetch_row($result);
$num_user_ids = $row[0];
Because $row[0] contains the first field in the row, which is 36 in my example.
As for the second part, you'll want to bind parameters, like this:
$stmt = $mysqli->prepare("SELECT COUNT(user_id) FROM users WHERE username = ?");
$stmt->bind_param('s', $username);
The s means that you're replacing the ? in the first row above with a string, which is given in the second argument.
You could also do what you do, but you'll have to surround $username with single quotes, because the query won't do it for you.

Answer 3

You are only getting one row from mysqli_num_rows in the first function because there is only one row in the result set (the row with the COUNT value in it). If you want to get the value you need to actually read the result set. Alternatively you could not use the COUNT function in the first case and then use mysqli_num_rows.
In the second function, my guess is that you need to add single quotes around the username value you are searching for. You should look at your actual errors returned from the query though and work from that.