匹配 mysql (php) 中的 email 地址不起作用 - 来自 paypal ipn

Question

Having a noob day.

For some reason this works:

// check for user in db
$result = $connector->query('SELECT username FROM wallart_users 
                             WHERE username ="test@testmail.com");

if (mysql_num_rows($result) == 0) {
  ////// ADD NEW USER

But this doesn't:

$payer_email = mysql_real_escape_string($payer_email);
// check for user in db
$result = $connector->query('SELECT username FROM wallart_users 
  WHERE username ='.$payer_email);

if (mysql_num_rows($result) == 0) {
  ////// NEW USER

The first one matches and sends me further down the page.
The second one doesn't match (with or without the escape) and tries to create a new user with the same credentials (then fails as the user already exists).
The email address PayPal is sending is always the same, and the database contains the right address.
I have written both addresses to a log file during the process and they look identical.

Answer 1

'SELECT username FROM wallart_users WHERE username ="'.$payer_email.'"';

Answer 2

This code

$result = $connector->query('SELECT username FROM wallart_users 
WHERE username ='.$payer_email);

Should be

$result = $connector->query("SELECT username FROM wallart_users 
WHERE username ='$payer_email' ");

Answer 3

Do you not think that you are doing some thing wrong with your second query? you should change your query like this

$result = $connector->query('SELECT username FROM wallart_users WHERE username ="'.$payer_email.'"');