Cannot write to file, although is_writable returns true

Anyone have any ideas on this? I check the file with file_exists()and is_writable(), both of which return true. I set the directory and the file in 777only to be safe, but try fwrite () or file_put_contents () do not change the file

Here is the relevant code:

$filePath = dirname( __FILE__ ) . '/' . 'myfile.txt';
file_put_contents($filePath, "waffles are delicious");