mysqli multi query separation - php

Question

I know you can run multiple queries with mysqli_multi_query but I have a problem :
For a registration page I want to perform 2 checks and then an insert :
Check1 = Does username exist?
Check2 = Email already been used?
If both checks are negative then do the insert query.
But how can I sepperate the errors ?
when a username exists it has to return 'Username already exists'
if email is already been used it has to return 'Email already in use'
Here is my code:
$sql = "check username query; ";
$sql .= "check email query; ";
$sql .= "Insert query";
if (mysqli_multi_query($conn,$sql)) {
do
{
if ($result=mysqli_store_result($conn)) {
while ($row=mysqli_fetch_row($result)) {
if (mysqli_num_rows($result) > 0) {
//DO STUFF
//Here do i need to return if the username or email exists or not.
};
mysqli_free_result($result);
};
} else {
echo 'Query fout!';
};
while (mysqli_next_result($con));
};
mysqli_close($con);

Answer 1

You do not write a ; after an if statement or a loop. Furthermore, your do {} while(); loop is not in the correct format.
Correct Format:
$sql = "check username query; ";
$sql .= "check email query; ";
$sql .= "Insert query";
if (mysqli_multi_query($conn,$sql)) {
do {
if ($result = mysqli_store_result($conn)) {
while ($row = mysqli_fetch_row($result)) {
if (mysqli_num_rows($result) > 0) {
//DO STUFF
//return if the username or email exists
}
mysqli_free_result($result);
}
}
else {
echo 'Query fout!';
}
} while (mysqli_next_result($con));
mysqli_close($con);
}

Answer 2

mysqli_multi_query() is not the tool to use for this.
That mysql[i] requires you to jump through hoops to execute more than one DML operation per call provides a lot of protection against sql injection (but it still falls a long way of a full solution for such attacks). There is no need to implement these as a multi-statement-query.
There is a performance and scalability benefit to limiting the number of round trips to the database - but you can do this:
SELECT SUM(IF('$username'=user.username, 1, 0)) AS usernames,
SUM(IF('$email'=user.email, 1, 0)) as emails
FROM users
WHERE username='$username' OR email='$email'
to get the results in a single query.
It would be even more efficient to not bother with a SELECT, but instead add a unique index to each of the attributes in the database then handle a duplicate record error if the INSERT fails although this does not make for such a nice user experience. OTOH it does prevent enumeration attacks against the database.