fopen() question

hello, i just recently started getting permission errors when writing
to a file that's been working fine for a couple weeks now. i have made
no changes to the file or folder permission settings and i was
wondering if anyone knew of any other factors that could cause these
types of errors below. thanks in advance for a help with this problem.
aaron;

$content = "my file content";

// create a filename variable with the .iif extension for Quick Books
$filename = "qbprod.iif";

// delete old file so a new file can be created
unlink($filename);

// create a new file
$handle = fopen("/home/mysite/www/qbdata/$filename", 'a');

// write the contents to the file
fwrite($handle, $content);

// have dialog box prompt the user to open or save the file
header("Location:
http://www.mysite.com/qbdata/get_file.php?filename=$filename";);

Warning: Unlink failed (No such file or directory) in
/home/www/mysite/qbdata/get_product_data.php on line 112

Warning: fopen("/home/goknobs/www/qbdata/qbprod.iif", "a") - Permission
denied in /home/www/mysite/qbdata/get_product_data.php on line 115

Warning: Supplied argument is not a valid File-Handle resource in
/home/www/mysite/qbdata/get_product_data.php on line 118

Warning: Cannot add header information - headers already sent by
(output started at /home/www/mysite/qbdata/get_product_data.php:112) in
/home/www/mysite/qbdata/get_product_data.php on line 121

.