Converting date inside loop breaks loop - php

Question

So I have a MySQL query feeding into an array to sanitize it to meet the needs of a graphing library. I tried to convert the hours, currently in 24h format, to a slightly prettier 12h format. But when I do, it kills the loop after one iteration, and so only one value gets fed to the graph (instead of values for the whole day). I'm very new to PHP, so I'm not sure what is breaking this. What is the proper way to accomplish what I need to do?
$data = $conn->query('SELECT HOUR( TIMESTAMP ) AS HOUR , COUNT( DISTINCT detected_key ) AS num_rows
FROM Visitors
WHERE TIMESTAMP >= NOW( ) - INTERVAL 1 DAY
GROUP BY HOUR( TIMESTAMP )
ORDER BY `id` ASC
LIMIT 0 , 24');
while($row = $data->fetch(PDO::FETCH_ASSOC)) {
$prettytime = DATE("g:i a", STRTOTIME($row['HOUR']));
$visit[$prettytime]= $row['num_rows'];
}
EDIT: I tried using DATE_FORMAT already, but that still just outputs as the 24h format. The only documentation I found on the MySQL website always had it right after SELECT.
SELECT DATE_FORMAT( TIMESTAMP, "%l:%i %a" ) , HOUR( TIMESTAMP ) AS HOUR , COUNT( DISTINCT detected_key ) AS num_rows
FROM Visitors
WHERE TIMESTAMP >= NOW( ) - INTERVAL 1 DAY
GROUP BY HOUR( TIMESTAMP )
ORDER BY `id` ASC
LIMIT 0 , 24
Here is the website where you can see the graph. The one using the query above is the top left graph.

Answer 1

This would seem to work rather nicely from a database field defined as
`thetime` timestamp NULL DEFAULT CURRENT_TIMESTAMP
Then using the MySQL DATE_FORMAT() and HOUR() functions :-
SELECT thetime, DATE_FORMAT(thetime, '%r') from test_table
Gives the results :-
2014-01-20 10:57:45, 10:57:45 AM
2014-01-20 23:59:31, 11:59:31 PM
Or
SELECT thetime, HOUR(DATE_FORMAT(thetime, '%r')) from test_table
Gives :-
2014-01-20 10:57:45, 10
2014-01-20 23:59:31, 11