Undefined Index when variable is present - php

Question

Code:
<?php
if (isset($_REQUEST['g'])) {
if ($_REQUEST['g'] == "Set") {
$g = strtolower($_REQUEST['g'])."_title";
}else{
$g = strtolower($_REQUEST['g']);
}
$result1 = mysql_query("SELECT LEFT($g, 1) FROM cards GROUP BY $g ORDER BY $g ASC");
while ($row = mysql_fetch_array($result1)) {
?>
<?php echo substr($row[$g], 0, 1); ?>
<?php
}
}
?>
Error Message:
Notice: Undefined index: set_title in E:\xampp\htdocs\zipdown\include\sort.php on line 11
What am i missing?

Answer 1

Don't forget to escape the value of $g if it isn't known to be Set.
$g = mysql_real_escape_string($g);
Alias the column created by LEFT() as $g, enclosed in backticks:
$result1 = mysql_query("SELECT LEFT(`$g`, 1) AS `$g` FROM cards GROUP BY `$g` ORDER BY `$g` ASC");
//----------------------------------------^^^^^^^
Otherwise, the column name returned by your mysql_fetch_array() call is something like:
$row['LEFT(set_title,1)']
Note on security:
Given that your database must have a finite set of column names, it is advisable to use a whitelist against which you check the value of $g:
// Assuming your columns are named title, author, etc...
if (in_array($g, array('title','author','set_title','publisher','whatever'))) {
// do the rest of your work
}
else {
// Invalid value to $_REQUEST['g']
}

Answer 2

You should give an alias name for LEFT($g, 1), or the key name in result is just lkie LEFT(set_title, 1) if $g is set_title.

Answer 3

Your $g has the value of set_title and your $row does not have an element at that index.
To be more clear, $row['set_title'] does not exist.

Answer 4

If you ask for $row['set_title'] but there is no such key set_title available in $row, PHP will raise a notice to let you know that it couldn't find anything. It will return a null value.
This is a notice, not an error. Generally speaking, notices only show up when PHP is set up for development; for production sites, notices are usually turned off.
Check out the documentation here if you want to silence notices:
http://www.php.net/manual/en/function.error-reporting.php
http://www.php.net/manual/en/errorfunc.configuration.php