PHP fopen contains unwanted content

Of course after hours of pondering this problem, the first comment on my question lead me to solve it immediately.

The problem was that, although I was including this code within its own function at the top of the page, I was calling it only if a certain flag was set in the $_POST array. I wasn't checking for the flag until the end of the PHP file. I moved that check before the function, and it worked.

The original question is below:

I'm trying to use the fopen() function in PHP to output a CSV file, and although it contains the data I want, it also contains the entire HTML structure of the page, as well as inline stylesheets, before the content that I actually want to output.

I'm using this code (from here) pretty much unchanged. I'm very unfamiliar with PHP streaming and output, so I started from what I hope was a firm foundation:

$fileName = 'somefile.csv';

header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header('Content-Description: File Transfer');
header("Content-type: text/csv");
header("Content-Disposition: attachment; filename={$fileName}");
header("Expires: 0");
header("Pragma: public");

$fh = @fopen( 'php://output', 'w' );

global $wpdb;
$query = "SELECT * FROM `{$wpdb->prefix}my_table`";
$results = $wpdb->get_results( $query, ARRAY_A );

$headerDisplayed = false;

foreach ( $results as $data ) {
    // Add a header row if it hasn't been added yet
    if ( !$headerDisplayed ) {
        // Use the keys from $data as the titles
        fputcsv($fh, array_keys($data));
        $headerDisplayed = true;
    }

    // Put the data into the stream
    fputcsv($fh, $data);
}
// Close the file
fclose($fh);
// Make sure nothing else is sent, our file is done
exit;

My assumption is that this example was intended to be included in its own external PHP file, but due to the constraints I'm dealing with, I'm trying to include it inline instead. I've mucked about with output buffering a bit with no positive results, but the PHP documentation on these is quite sparse, so there's probably something I'm missing.

Solution

Of course after hours of pondering this problem, the first comment on my question lead me to solve it immediately.