MySQL/PHP database value check not allowing for extra characters

I have a username check in PHP, and whenever someone enters the value 'username2' when 'username' already exists, it returns that it is already taken.

    include 'connect.php';

$username = $_POST['r_username'];
$password = $_POST['r_password'];
$email = $_POST['r_email'];

$开发者_开发百科qry = "SELECT * FROM users WHERE 'username' = '$username'";

if(mysql_query($qry) != ''){
    echo "<script type='text/javascript'>
    alert('This username has already been taken!');
    window.location = 'register.php';
    </script>";
}else{...

Thanks for your help!

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$qry = "SELECT * FROM users WHERE 'username' = '$username'";

I don't know much about php or MySql, but just wanted to ask if the field_name should be 'username' or without quotes. May be its taking it as value instead of column_name. I'm not sure

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You are using mysql_query() incorrectly. It returns a resource to be used in other functions.

Read the manual: http://php.net/manual/en/function.mysql-query.php

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$query_response = mysql_query($qry)
if (mysql_num_rows($query_response) > 0) {
echo "<script type='text/javascript'>
    alert('This username has already been taken!');
    window.location = 'register.php';
    </script>";
}
else {
echo 'This username has not been taken yet. Please, proceed';
}

Take a look what mysql_query returns. Your condition (mysql_query($qry) != '') will always be true

Also, use backticks instead of apostrophes (') to enclose column names in queries. Now you comparing whatever $username string is with string username, but not a username column.

BTW, I agree with the others. Your query is not protected against SQL Injections. Use mysql_real_escape_string like so:

"SELECT * FROM users WHERE `username` = '".mysql_real_escape_string($username)."'";