PHP code for inserting non-duplicate values into MySQL - php

Question

I have basic PHP/MySQL experience, having taken an introductory class. My knowledge is literally limited to the following PHP codes:
if(!($stmt = $mysqli->prepare...)
if(!($stmt->bind_param...)
if(!$stmt->execute...)
I'm currently trying to write a program that allows a user to enter a new password, and checks the password against existing passwords in the database.
Here is what I have:
<?php
foreach($Password){
$dupesql = "SELECT PasswordID Passwords WHERE (Password = '$Password')";
$duperaw = mysql_query($dupesql);
if(mysql_num_rows($duperaw){
echo nl2br("$Password has already been used \n");
}
else{
echo "Password added \n";
}
}
?>
I got the code from this post: Check for duplicates before inserting
I'm not sure if the code itself has problems or if I need to add anything else to my PHP code to get this working, as it's currently producing an "Error 500".

Answer 1

MySQL extension is deprecated and probably you have PHP 7.0 from where it is removed. Rewrite your code to MySQLi or PDO. Check this question on how to convert to MySQLi: How could I change this mysql to mysqli?
Also, your code just doesn't add a password (never). Probably you expect to add it before the "Password Added" message, but be aware: the solution you want to use is not ideal, because there is a risk of race condition between checking the password for existence and adding it. This means that it is possible to add a password twice.
To solve this problem, you might want to use transactions. More details are covered in this question: PHP + MySQL transactions examples

Answer 2

I decided to go an entirely different route, which is to set the Password column as unique.
Then I did a simple INSERT that would prompt an error if the user attempts to add a duplicate:
<?php
if(!($stmt = $mysqli->prepare("INSERT INTO Heroes(HeroName, FirstName, LastName, Age, HomeCountry, RoleID) VALUES (?,?,?,?,?,?)"))){
echo "Prepare failed: " . $stmt->errno . " " . $stmt->error;
}
if(!($stmt->bind_param("sssisi",$_POST['HeroName'],$_POST['FirstName'],$_POST['LastName'],$_POST['Age'],$_POST['HomeCountry'],$_POST['RoleID']))){
echo "Bind failed: " . $stmt->errno . " " . $stmt->error;
}
if(!$stmt->execute()){
echo "Execute failed: " . $stmt->errno . " " . $stmt->error;
} else {
echo "Added " . $stmt->affected_rows . " row to Heroes.";
}
?>