File_put_contents returns true, but file doesn't write either.

I have a small PHP script that runs when I click a button on some HTML page:

<?php
if(isset($_POST['field1'])) {
    $data = $_POST['field1'] . "\n";
    $ret = file_put_contents('/tmp/mydata.txt', $data, FILE_APPEND | LOCK_EX);
    if($ret === false) {
        die('There was an error writing this file');
    }
    else {
        echo "$ret bytes written to file";
    }
}
else {
   die('no post data to process');
}

      

        
        
        
      

    

When I give html some text and run the script, I get an echo:

16 bytes written to file

However, when I then open the file, it is empty and reads as "zero bytes" in size.

I tried communicating with permissions and checking the log files without prompts, I made sure the private text gets to the php script. I'm completely stumped, any suggestions?