How to debug PHP database code? - php

Question

I am new in PHP and need help with my below code. When I am entering wrong userid instead of giving the message "userid does not exist" it is showing "password/id mismatch. Please guide me where I am wrong.
<?php
session_start();
$id = $_POST['userid'];
$pwd = $_POST['paswd'];
$con = mysqli_connect("localhost", "????", "????", "??????");
if ($con) {
$result = mysqli_query($con, "SELECT * FROM users WHERE userid=$id");
if ($result) {
$row = mysql_fetch_array($result);
if ($row["userid"] == $id && $row["paswd"] == $pwd) {
echo "Welcome! You are a authenticate user";
if ($id == $pwd)
//my default login id and password are same
{
header("Location: changepwd.html");
} else {
header("Location: dataentry.html");
}
} else {
echo "ID/Password Mismatch";
}
} else {
echo "User does not Exist !!!";
}
} else {
echo "Connection failed - ".mysqli_error()." -- ".mysqli_errno();
}
?>

Answer 1

The main problem you have is that you're mixing up between the mysqli and mysql functions. These two libraries are not compatible with each other; you must only use one or the other.
In other words, the following line is wrong:
$row=mysql_fetch_array($result);
It needs to be changed to use mysqli_.
While I'm here, going off-topic for a moment I would also point out a few other mistakes you're making:
You aren't escaping your SQL input. It would be extremely easy to hack your code simply by posting a malicious value to $_POST['userid']. You must use proper escaping or parameter binding. (since you're using mysqli, I recommend the latter; it's a better technique).
Your password checking is poor -- you don't appear to be doing any kind of hashing, so I guess your passwords are stored as plain text in the database. If this is the case, then your database is extremely vulnerable. You should always hash your passwords, and never store the actual password value in the database.
I've gone off topic, so I won't go any further into explaining those points; if you need help with either of these points I suggest asking separate questions (or searching here; I'm sure there's plenty of existing advice available too).

Answer 2

else
{
echo "ID/Password Mismatch";
}
is connected with the
if($row["userid"]==$id && $row["paswd"]==$pwd)
{
So since you are giving a wrong id. It echo's: ID/Password Mismatch
Also the else at if ($result) { wont ever show since
$result = mysqli_query($con, "SELECT * FROM users WHERE userid=$id");

Answer 3

You need some additionnal checks:
select * return 1 row (not 0, and not more)
you need to protect the datas entered by the html form (for example someone could enter 1 or 1 to return all rows
<?php
session_start();
$con = mysqli_connect("localhost", "????", "????", "??????");
$id = mysqli_real_escape_string($_POST['userid']);
$pwd = mysqli_real_escape_string($_POST['paswd']);
if ($con) {
// don't even do the query if data are incomplete
if (empty($id) || empty($pwd)
$result = false;
else
{
// optionnal : if userid is supposed to be a number
// $id = (int)$id;
$result = mysqli_query($con, "SELECT * FROM users WHERE userid='$id'");
}
if (mysqli_num_rows($result) != 1)
$result = false;
if ($result) {
$row = mysqli_fetch_assoc($result);
if ($row["userid"] == $id && $row["paswd"] == $pwd) {
echo "Welcome! You are a authenticate user";
if ($id == $pwd)
//my default login id and password are same
{
header("Location: changepwd.html");
} else {
header("Location: dataentry.html");
}
} else {
echo "ID/Password Mismatch";
}
} else {
echo "User does not Exist, or incomplete input";
}
} else {
echo "Connection failed - " . mysqli_error() . " -- " . mysqli_errno();
}
?>

Answer 4

Try with isset() method while you are checking if $result empty or not.
that is in line
if ($result) {.......}
use
if (isset($result)) { .......}

Answer 5

$result is always true, because mysqli_query() only returns false if query failed.
You could check if $result has actual content with empty() for example.

Answer 6

You can use this sql compare password as well with userid
$sql= "SELECT * FROM users WHERE userid='".$id.", and password='".$pwd."'";