To go with signup.php, Example 21-6 presents checkuser.php, the program that looks up a username in the database and returns a string indicating whether or not it has already been taken.
Because it relies on the functions sanitizeString and queryMysql, the program first includes the file
functions.php. Then, if the $_POST variable 'user' has a value, the function looks it up in
the database and, depending on whether it already exists as a username,
outputs either “Sorry, this username is taken” or “This username is
available.” Just checking the function mysql_num_rows against the result is sufficient
for this, as it will return 0 if it is
not found, or 1 if it is.
The HTML entities ✘
and ✔ are also used to
preface the string with either a cross or a check mark.
<?php // checkuser.php
include_once 'functions.php';
if (isset($_POST['user']))
{
$user = sanitizeString($_POST['user']);
if (mysql_num_rows(queryMysql("SELECT * FROM members
WHERE user='$user'")))
echo "<span class='taken'> ✘ " .
"Sorry, this username is taken</span>";
else echo "<span class='available'> ✔ ".
"This username is available</span>";
}
?>